TB6612FNG Motor Control Board

Background

As part of a to-be-announced project I am working on, I ended up designing a new Toshiba TB6621FNG motor control board which is designed to be compatible with the Raspberry Pi. Before I began this design, the lab I am working in had been using controllers based on the Rohm BD6222HFP chip. This is a chip which is well suited to hefty 12V designs. However, we now need something more efficient and less expensive. The TB6612FNG chip is:

    • Less expensive (by ~$1 at single unit scale)
    • Requires more hobbyist friendly supply voltages ( 3-5.5V instead of 6-18V)
    • Provides similar output amperage compared to the Rohm chip.
    • Input current in standby (IIH) averages 35μA less.

    All this lead me to design around the new chip. Of course, breakouts from the usual suspects such as Sparkfun (seen here) and Adafruit (seen here) are available. Adafruit even provides an advanced version which is (in my opinion) over-specialized (see it here). So why did I reinvent the wheel in this case? Because the basic breakout boards are too limited and the advanced version is too limiting.

    Pi Woes

    The GPIO pins on the Raspberry Pi are a wonderful thing. They provide so much room for innovation. However, when you only need one or two specific pins, and those pins are scattered all over the GPIO, it is inconvenient and slightly expensive to design a board with the full 26 pin socket. Instead, selective use of 3-pin servo headers can be used at lower cost. The TB6612FNG motor control board I designed uses this type of design.

    Additionally, there is a problem unique to certain pins of our good ol’ GPIO friend. During boot, pins are not consistently pulled up or down. This can mean that a motor run by these pins may do strange things during boot. If one of those motors is controlling some sort expensive optics as in my case, this is to be avoided. There are a few work around options for this. It is possible to set up a cron job at boot which will set all of the pins to pull up or down, but this may be a challenge to a user with less bash experience. For my project, I need something that allows a user to run the device without worrying about the details. So I implemented a hardware solution. The GPIO pins are pulled down by 100k resistors on my TB6612FNG motor control board.

    An advanced setting was made available to divorce the chip ground from the motor power ground by way of a solder jumper.

    Schematics

    Pictured below is the schematic for my TB6612FNG motor control board. The project was designed with KiCAD.

    TB6612FNG motor control board schematics

    The picture below shows the layout of the board, omitting the back copper plane. I was able to fit all of the traces on one side of the board, so the B.Cu layer is mostly ground plane thermal relief.

    TB6612FNG motor control board copper layout

    Next week, I will show how I made the board.

Okstate LaTeX Dissertation Template

Dissertation Publication

Since my dissertation was recently published (see last weeks post), I thought I would post the template I used to create it. Oklahoma State University does not currently offer a default LaTeX template for a dissertation. Instead, they only offer a Word template (gross!). I took it upon myself to make an Okstate LaTeX dissertation template.

Okstate LaTeX Dissertation Template

Lofty Goals

If you were putting off learning LaTeX, but have a big upcoming task such as a dissertation to write, I strongly encourage you to try it. I hope that by making my Okstate LaTeX dissertation template available to everyone, life could get a bit easier.

Please note that this template is unofficial, so I make no claims that this 100% meets the standards of the graduate college. However, the Okstate LaTeX dissertation template I created was accepted for publication with no issues. I have attempted to make this available through official channels. Sadly, the graduate college matriculation department has not replied to my contact attempts.

Github Link and Invitation

Find the link to the template below. If you have any changes or comments, please feel free to contact me. Additionally, forks of this work are encouraged to improve the material for future students. Outreach and publication are at the core of a doctoral degree!

LINK TO THE TEMPLATE

Dissertation: Development and Applications of Chemical Sensors for the Detection of Atmospheric Carbon Dioxide and Methane

It is a new year, and my dissertation has finally made it past embargoes to publication. Allow me to present: Development and Applications of Chemical Sensors for the Detection of Atmospheric Carbon Dioxide and Methane. You can view the ProQuest entry here. There is a download link to the full text at the bottom of this entry.

Title

Development and Applications of Chemical Sensors for the Detection of Atmospheric Carbon Dioxide and Methane

Abstract

This is a description of the design of a low-power, low-cost networked array of sensors for the remote monitoring of carbon dioxide and methane. The goal was to create a scalable self-powered two-dimensional array for the detection of these gases in a large area. The sensor selection, electronic design, and data communication was studied and optimized to allow for multiple units to form a self-assembling network for acre-scale coverage with minimal human intervention. The final electronic design of the solar-powered units is flexible, providing a foundation for future field deployable remote monitoring devices. Sensors were selected for this application from commercially available models based on low-power, low-cost, market availability, detection range, and accuracy around the global baseline criteria. For environmental monitoring, carbon dioxide sensors are characterized near 400 ppm and methane from 2 to 200 ppm. For both gases, exertions up to several 1000 pm were examined to mimic large releases. An Xbee mesh network of radios was utilized to coordinate the individual units in the array, and the data was transferred in real-time over the cellular network to a dedicated server. The system was tested at a site north of the Oklahoma State campus, an unmanned airfield east of Stillwater, OK, and an injection well near Farnsworth, TX. Data collected from the Stillwater test sites show that the system is reliable for baseline gas levels. The gas injection well site was monitored as a potential source of carbon dioxide and methane leaks due to the carbon dioxide injection process undertaken there for carbon sequestration and enhanced oil recovery efforts. The sensors are shown to be effective at detecting gas concentration at the sites and few possible leak events are detected.

Reference Information

Subject Classification 0485: Chemistry
0486: Analytical chemistry
0494: Physical chemistry
Identifier / keyword Pure sciences
Carbon dioxide
Detection
Enhanced oil recovery
Methane
Remote monitoring
Sensing
Author Honeycutt, Wesley T.
Number of pages 196
Publication year 2017
Degree date 2017
ISBN 9780355396188
Dissertation/thesis number 10276467
ProQuest document ID 1965485293
Advisor Materer, Nicholas F.
Committee members Apblett, Allen W.
Fennell, Christopher J.
Ley, M. Tyler
White, Jeffrey L.
University/institution Oklahoma State University
Department Chemistry
University location United States — Oklahoma
Degree Ph.D.

Download

Here is a download link to the complete dissertation file: Dissertation.pdf

NOTE: This file is a 166.7MB PDF document. Since I am not a rich man, my website is hosted on a slow, inexpensive server. This download may take some time. It may be quicker to download from a library site, but I want to make sure the document is available to everyone.

Greek Philosopher Mask 3

Two weeks ago, I discussed the inspirations for a Greek Philospher mask. Last week, I showed how this mask was built up. This week, my Greek Philosopher mask will be completed with the addition of paint and lacquer.

An Aside About Rocks

The ‘stone’ surface of the Greek Philosopher mask was painted to look similar to marble. To do this, I first set about finding what sort of marble I wished to imitate. In keeping with the faux-historicity of the mask, I decided to base the colors in the marble off of that which was available in the Hellenistic period. Additionally, I wanted to give the stone an aged appearance which fits with the thematic elements discussed in the inspirations post. My Greek Philosopher mask was to be made of material which imitates discolored Parian marble. Marble from the island of Paros is recognizable as a uniform, crystalline white marble (Washington 1898). The picture below depicts a magnified image of Parian marble.

From Washington 1898.  I wonder if that arrow is part of the original?

While no marble ages the same, generally Parian marble ages in such a manner to produce a distinctly orange-hued discoloration. This is possibly due to sub-surface oxalate migration or formation (Pinna 2015). Below are some pictures of discolored Parian marble. The first is a bust of Alexander. The second is Nike of Samothrace.

Bust of Alexander

Nike of Samothrace

The hues present in Parian marble statues is distinct from other sources from the area such as Carrara marble. This is the material used in several recognizable sculptures, such as Michelangelo’s Pietà (pictured below) and the Elgin Marbles. These figures often take on a more gray-hued discoloration with age. This is possibly due to fungal and/or bacterial growth on the surface (Marvasi 2008, Konkol 2009).

Michelangelo's Pietà

Stone in Paint

To produce the marble effect I wanted, I used a variant of a technique called Scagliola. To employ this traditional method for my modern Greek Philosopher mask, I mixed white latex paint with plaster of paris and acrylic paints which acted as pigments. While the surface was wet with the paint, I drew in paints of another pigment to form lines. Then, the light tone of the first few coats was given a dabbed on patina with warm-toned grays and browns to mimic approach the aged Parian marble look I wanted. Finally, carbon soot was rubbed on the dried paint.

After the painting was complete, I added highlighting by marring the surface. A nail was used to scratch the painted surface down to the bottom layers of the scagliola (beneath the dabbed gray, brown, and carbon). These scratches were added in the recessed corners of the surface such as the beard and hair. Additional scratches were added in a haphazard fashion to the smoother surfaces. By using the scratches on the surface, a more aged look is achieved. Applying the scratches to recesses of the surface of the mask mimics the patterns of marble which has been discolored due oils from the skin rubbing on the surface.

Greek Philosopher mask with painted and scratched surface

Sealed and Shined

As the Greek Philosopher mask was intended to be functional, the completed surface needed a sealant to prevent the paint from being removed or damaged during wear. Several coats of a water-tight lacquer was applied to the mask. This would prevent spills or rain from damaging the surface, and it would prevent sweat from damaging the interior. The lacquer I used gave the mask the appearance of a polished sheen. If I was to attempt this again, I am not certain I would use a glossy lacquer, as a matte coat might have been more appropriate for a stony surface.

A shiny lacquer coating

The Greek Philosopher Mask used to play Hades

The complete mask was officially shown to the public at Halloween party. Accommodating the wish of my date for a couple’s costume, I dressed as Hades and she as Persephone. The entire costume was a hit, the mask especially. I admit, I still wear the mask from time to time. Since this is one of my favorite works, I find it hard to resist popping it on for a good session of pondering.

Hades and Persephone pose at a party.  Remember to never drive drunk, always have Charon give you a ride back across the river.

Works Cited

Washington, Henry S. “The Identification of the Marbles Used in Greek Sculpture.” American Journal of Archaeology, vol. 2, no. 1/2, 1898, pp. 1–18. JSTOR, JSTOR, www.jstor.org/stable/496773.

Pinna, D., Galeotti, M. & Rizzo, A. herit sci (2015) 3: 7. https://doi.org/10.1186/s40494-015-0038-1

M. Marvasi, F. Donnarumma, A. Frandi, G. Mastromei, K. Sterflinger, P. Tiano, B. Perito, Black microcolonial fungi as deteriogens of two famous marble statues in Florence, Italy, In International Biodeterioration & Biodegradation, Volume 68, 2012, Pages 36-44, ISSN 0964-8305, https://doi.org/10.1016/j.ibiod.2011.10.011.

Nick Konkol, Chris McNamara, Joe Sembrat, Mark Rabinowitz, Ralph Mitchell, Enzymatic decolorization of bacterial pigments from culturally significant marble, In Journal of Cultural Heritage, Volume 10, Issue 3, 2009, Pages 362-366, ISSN 1296-2074, https://doi.org/10.1016/j.culher.2008.10.006.

Greek Philosopher Mask 2

Last week, I discussed the inspiration and first few layers of my Greek Philosopher mask. This week, I will be showing a bit more of the build process.

Building Up

The process I used to make my Greek Philosopher mask is very similar to what was used in the Witch mask design. The bulk of the body of the mask was constructed from my special paper mache material composed of shredded and chopped paper, plaster of paris, PVA glue, and water. The material is layered on in a rather slow process, considering the lengthy drying times for the thick material. Between each layer, I coat the mask in Mod Podge PVA glue to provide extra strength.

Greek Philosopher mask? More like fuzzy sheep face mask.

The Greek Philosopher mask is thickened with each layer, and elongated at the same time as the beard is constructed. The original holes cut for the eyes are over-expanded to give ample viewing room for the wearer and give room for a upper cheekbones. The nose is broadened from the cranial ridge from the first few layers. The cranial ridge is a design consideration to give additional strength to the build. A mouth is cut, and material is added to the edge to form the lips and mustache. The lips are flared outwards to mimic the shapes used in traditional Greek masks. This flared end is intended to increase the intensity of a sonorous voice for theater that predates many auditory enhancements we see today.

These lips are made for loudness.

The top of the head is built up with more material to give the illusion of voluminous hair in the finished piece. The Greek Philosopher mask is given an intense demeanor with the shape of material added to the forehead and eye sockets. The mask’s character is given heavy eyebrows and deep-set cheekbones to that effect. The shape of the eyes is triangular to give a more threatening or admonishing tone to the character.

Sanding Down

Finally, the mask is built up in the manner which I want. The surface is rough and oddly colored, but expression and characteristics for my Greek Philosopher mask are set. The next step is to sand out many off the imperfections to prepare for more detailed work. Since the material is composed of plaster, I would prefer not to inhale the dust. Sanding was performed outside on a day with an adequate breeze.

Taking shape and growing hair

After the mask was sanded, more material was added to coat the smoothed surface. This time, I used pure plaster of paris to form the more detailed and smooth bits. A watery mix of plaster was added to the face and beard, giving a smooth, finished look.

The hair was constructed from a dryer plaster mix. Strands of hair were shaped with a nail from this more solid mixture. The Greek Philosopher mask was given wavy hair. Many of the original Greek masks have curled hair to fit with the traits of the people in the region, but I opted to not use that. The logic is that the character is a hoary old thinker with less concern for his appearance. The weight of years of extra hair growth while consumed by meditation and thought would have naturally straightened the hair of the individual.

Finally, everything was sanded down again with a finer grained sandpaper to prepare the surface for paint.

The

Next week, I will be showing the painting inspirations and methods to produce an appropriate look for the Greek Philosopher mask and finishing the build.

Greek Philosopher Mask 1

One of my favorite creations of late has been my Greek Philosopher mask. For the next three posts, I’m going to go over the construction process I used for it.

The Madness

The inspiration for the design of the Greek Philosopher mask came from three sources. The first source, and perhaps the most subtly influential one, is The Frogs by Aristophanes. From the wikipedia article, the plot centers around “the god Dionysus, who, despairing of the state of Athens’ tragedians, travels to Hades (the underworld) to bring the playwright Euripides back from the dead. (Euripides had died the year before, in 406 BC.) He brings along his slave Xanthias, who is smarter and braver than Dionysus.” The play is a comedy of language, as many of the period were, and pulls weighty issues to light with a certain levity. I had the opportunity to play a portion of this back in the day with my dramatic other half. As such, I have fond memories of playing Dionysus against my matched foil. This inspiration is something which, I suspect, hid beneath my conscious thought as I came across my other sources.

cover of Stone's Reach by Be'lakor

Near the time of its remastered release, I became enraptured by the album Stone’s Reach by Be’lakor. I was still delving into the melodic death metal scene at that time, and this album had a profound effect on my current musical preferences. The lyrical content of the songs varies, and no cohesive theme runs through the complete work. However, the mind is a beast of patterns. Where there are stripes, reason first seeks a tiger. Lyrics I heard (and occasionally misheard) led me to find commonalities among other songs on the album, and a personal narrative was formed. This was influenced by the album cover, pictured above, which is a purposefully framed shot of the classic sculputre “Perseus with the Head of Medusa“. With my head in a darkened world of chthonic crypts and my eyes pointed toward the perfectly-hewn form gracing the front, I heard the essential lyric:

“The carvings have outlived the hand
Which bled to first begin them”

The final bit of inspiration for the Greek Philosopher mask was taken from an reproduction Greek “philosopher” mask. I originally went on a Google spree looking for the right work to inspire mine. There are some lovely collections out there of originals, such as the British Museum. You can view the pieces online, such as the ones pulled out by the linked advanced search. I ended up being attracted to a modern reproduction from a (now defunct) website. It can still be accessed via The Wayback Machine. The mask in question was entitled “Philosopher”, whence mine inherited the name.

Greek Philosopher mask reproduction which was one of the important inspirations for this project.

The Make

The first layer of the Greek Philosopher mask was produced in a bowl. The interior was contoured in such a way that it would roughly match my facial features. This method differs from the mask creation detailed in the Witch project. In the Witch mask, the back side was built upon from a flat surface, with less regard to a good fit against the face. For the Greek Philosopher mask, I wanted something which would fit well for long periods with heavy movement, similar to the expectations of a good theater mask.

After a bit more material was added to the face, a head covering was added. Simple eye slits were cut which would later be broadened with further building.

The beginning of the build

Next week, I will cover the construction a bit more, and the mask will begin to resemble it’s final form.

SiC Logic: A Rejected Proposal

This is a proposal I wrote and submitted to the July 2017 cycle of the NASA NPP program to work on development of SiC logic for a Venusian satellite project.. It was rejected and I went another direction. The SiC logic proposal idea was generally well received. While many of the reviewers comments could have been addressed in a treatise with ample page limits, there was little I could do about the primary concern. It is clear that the reviewers would have been more comfortable awarding the postdoctoral position to someone who had worked with silicon carbide before, rather than explaining in theory and going for on-the-job experience. This is a fair assessment that I suspect I will never be able to address, as SiC logic is not a common enough field to get easy exposure too.

So with the understanding that I will likely not be able to work on this in the future, I would like someone else to pick up the reins and move forward with the project. This is a very important project that has many applications, as will be addressed below. I urge readers to STEAL THIS WORK. Therefore, I am taking the time to release this rejected proposal on my site to allow others to learn from it and, hopefully, continue the proposed project. The full text of the project is below, modified from the original to fit the current medium.

As always, feel free to contact me for info or to chat about this.  My contact info can be found on the CV page.


Development of Logical Circuitry using Silicon Carbide Substrates for Control Circuits in Harsh Environments

by

Wesley T. Honeycutt

Title of Research Opportunity: SiC Circuit and Device Design and Fabrication

NASA Center: Glenn Research Center

1. Statement of problem

Recently, NASA has begun prioritizing an exploratory mission to Venus [1], including a surface lander [2]. Previous landers from the Venera program by the USSR were only able to last, at most, two hours on the unforgiving Venusian surface [3]. The extreme temperatures and oxidative atmosphere of the planet’s surface mean it is very difficult for current electronic systems to perform for extended periods. While a wealth of new high temperature and pressure sensing instruments have been developed as part of the Venus Exploration Analysis Group (VEXAG), the glue electronics to control and operate such a lander are still elusive [4].
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The development of silicon carbide (SiC) semiconductor devices has gained increasing popularity in the past 20 years, yet the technology lacks the wide breadth of available devices which competing substrates currently offer. Far from the first light emitting diode [5] [6] which was originally made in SiC, today SiC can be found in many high power applications. Recent developments have even allowed the creation of power switching devices with high-voltage junction temperature tolerance up to 1200 V in both JFET and MOSFET designs [7] [8]. Current SiC logical devices are produced using JFET technology. Core logic gates such as the inverter, NAND, and NOR have been demonstrated in both depletion mode [9] and enhancement mode [10] SiC JFET. Despite recent demonstrations of a ring oscillator and development of 100’s scale transistor level logic using SiC JFET technology [11], there is a dearth of advanced logic designs for SiC circuits which would further the NASA Venus lander objectives. The goal of this proposed work is to develop discrete digital logic devices using SiC substrates and patterning.

2. Background and relevance to previous work

Silicon carbide has several interesting properties that make it an attractive semiconductor platform for devices intended to withstand harsh environments. At standard atmospheric pressures, SiC remains crystalline with no defects at temperatures up to 2760 °C before partial sublimation [12]. At higher pressures, this temperature stability increases. Under 100 atm, similar to the pressure at the surface of Venus, SiC remains solid up to 2830 °C before melting. Individual junctions of p-doped and n-doped SiC material have a much higher tolerance for heat than more commonly used substrates such as Si. This is because the optimum working temperature of SiC is very close to the highest normal mode vibrational temperature, or band gap [13]. Microelectromechanical systems (MEMS) based on SiC have been established as a viable device for some time [14], and some developed MEMS devices are pushing temperature limits in sensors [15]. Logic gates manufactured from SiC can take advantage of this property by creating devices which tolerate high voltage switching, a task which produces a large amount of heat [16]. The converse can be true as well: it is possible to make low power logic gates that tolerate extreme temperatures.

Additionally, SiC has been shown to be particularly resilient against radiation damage. Circuits made of SiC have been reported to exhibit much lower rates of carrier damage due to deep material defects caused by neutron flux, with McGarrity et al. reporting 1/3 of the damage compared to Si based devices at doses of 1016 neutrons/cm2 [17]. This makes SiC technology attractive for radiation heavy environments, including satellites and exploratory vehicles. The large gate sizes in current SiC technology compared to modern CMOS feature sizes makes the SiC further resistant to radiation effects [18]. If this material is used for the sensitive electronic equipment, it is possible to have reduced failure rates with current design specifications. Furthermore, it is possible that future devices using this technology may require less device redundancy for radiation hardening purposes, making it possible to create exploratory vehicles with greater computational power and reduced power consumption.

Logical units for SiC are designed by the NASA Silicon Carbide Electronics and Sensors group using depletion mode JFET technology on both 4H-SiC and 6H-SiC wafers. At the simplest level of description, an n-type JFET, the most durable and stable type for high temperature operation [11], is a transistor gate with the drain and source connected to an n-type substrate with a p-type gate separated by a depletion layer. A detailed diagram of the current archetypal SiC JFET layers can be found in the literature [19]. These JFETs are normally ON devices which switch OFF when a voltage applied to the gate passes a certain threshold in a reverse biased regime. Due to this design, the source requires a negative voltage, making the HIGH and LOW bits ground and a negative voltage, respectively. As certain parts of a logical array require a positive voltage, a JFET logic circuit is necessarily more complex to design due to the presence of an additional power rail. Further complications arise from the body bias effect based on the substrate voltage [20] and radially dependent resistivity observed in printed wafers [19].

As a physical chemist, I have an outside perspective on device function at the gate interface level which allows for a critical approach to the challenges posed by innovative logic design. My previous experience with silicon based control systems has demonstrated that I am capable at computer aided design (CAD) of electronic systems [21]. In addition to my design experience, I have also used modeling programs such as SPICE to characterize the behavior of a Fourier transform device for monitoring sample conductivity. Modern developments in microcontroller prototyping boards allow for integration of one device to test another. My previous work gave me experience with prototype evaluation of electronic systems and bringing a scientific approach to an engineering problem. Particularly, previous designs for sensors deployed in unpredictable climates have made me conscious of the need for devices built to tolerate these conditions including temperature dependent conductivity sensors [22]. As a scientist who enjoys developing devices for monitoring and evaluating physical and chemical properties, the challenges posed by SiC JFET design appeal to my desire to broaden my skills.

3. General methodology, procedures to be followed, and timeline for completion of each step

The goal of this proposed work is to develop logic circuits required for a low power monitoring device tolerant to harsh conditions by application of known computing architecture to the emerging silicon carbide technology. At the highest level of abstraction, a device of this nature requires a central processing unit (CPU), level shifters, analog to digital converters (ADC), switched power supply units (PSU), and memory. Certain portions of this list, including level shifters [9], ADC [23], and PSU [24], have been demonstrated in previous research. In the case of a modern application, a single package microprocessor can handle most of these tasks. However, minimal work has been done creating complex logic for the CPU in SiC. The development of this device must start from a basic level, designing the subcomponents of the CPU individually.

A CPU can be generalized as a combination of individual logic units, primarily these are registers and arithmetic logic units (ALU). The goal of this proposal is to develop the technology in SiC JFET for both of these logic units to create a functional 8-bit microprocessor.

3.1 Register

Figure 1

Decreasing level of register architecture in SiC logic

This is an example register design showing the design hierarchy. Part 𝒶 shows the primary user accessible 8-bit registers included in the design of the popular Intel 8085 chip [25]. Part 𝒷 shows detail of one register from part 𝒶 [26] [27]. Each bit of the register is a flip-flop which adjusts the value from D to Q for each bit on in a clock cycle. Part 𝒸 shows one of these flip-flops. A flip-flop is composed of two latches, a master and a slave, which copy D to Q on the rising edge of the clock cycle. This saves the state of the flip-flop at all other times. Part 𝒹 shows one of the latches, a simple logic circuit consisting of one NOT, two ANDs, and two NORs in a bistable arrangement. Part ℯ shows a diagram of one CMOS NOR gate depicted in part 𝒹 as they would be designed for patterning on the SiC surface using the program MAGIC [28]. In short: multiple of ℯ make up 𝒹, which make up 𝒸, which make up 𝒷, which make up 𝒶.

The register is a logic unit which is included in many important CPU and device components such as the primary register, memory, instruction set, and program counter. To construct this unit, we must reduce this piece down to its relatively simpler components. The register is a series of clock synchronized flip-flops, a flip-flop is a pair of latches, a latch is simply a pair of NOR gates connected in a certain manner, and a NOR gate is constructed from a few individual transistors. Figure 1 shows the decreasing levels of abstraction from the register of a popular integrated circuit to the layers and doping of a single NOR gate designed for CMOS technology. The transistor technology for JFET SiC gates have been previously demonstrated for NOR gates, making the technology scale up possible.

Due to the synchronous nature of the register and the importance of timing in computing, each device will be rigorously analyzed as it is created and tested. As it is impractical to directly measure single gate delay, a value which is often on the scale of picoseconds, it is useful to estimate the rise and fall of a signal through a gate using computational models and values of capacitance and current for each interconnect [29]. The delay for multiple equivalent gates is additive at scale, making the averaged delay over multiple gates in a device a calculable quantity with simple instrumentation. The averaged gate speed can be measured by construction of an array of latches acting asynchronously. This will be used to verify computationally calculated values.

Keeping with the “bottom-up” approach to design on the SiC substrate, the first task will be designing and verifying a single bit flip-flop, which has not been reported in literature. The patterned device will be characterized by tests to determine the ability to retain and replace the bit over multiple clock cycles. As SiC devices are being sought to operate in harsh conditions, tests will be undertaken to determine the bit retention ability of the flip-flop at a range of temperatures from standard conditions to >500 °C. Testing of the device can take place with a voltage monitor (capable of determining bit state) at short time scales and a local RC oscillator (as oscillators must be close to the circuit to prevent return current, making an external crystal oscillator impractical for high temperature tests). The second task will be to scale this device up to a full 8-bit register. Findings from the first task will be used to determine the design of this circuit, and it will be tested and verified in a similar manner.

3.2 Arithmetic Logic Unit

Another essential component of the CPU is the Arithmetic Logic Unit (ALU). This is an umbrella term for portion of the processor which performs simple arithmetic operations such as addition, subtraction, multiplication, division, and comparison. Each of these operations requires a distinct circuit. The addition and subtraction circuitry will be designed in the form of a carry-lookahead adder (chosen for the ability to perform faster calculations than many other adder circuits, a property which will be valuable in lifetime-limited missions), using two’s compliment values for subtraction operations. The multiplication and division operations will be performed on circuitry designed using logical and arithmetic shifters. Equality comparators are simple ANDs of a number of XNOR gates, which can be constructed from the NAND and NOR gates available in SiC JFET. Magnitude comparators are a variant of subtractors. All of these have well documented logical designs which can be applied to SiC systems.

Simple arithmetic circuits are used throughout the computer system. For example, operations are performed based on orders delivered by Operation Codes (Opcodes) from the program register. These instructions must be interpreted by the logic of the device by arithmetic means. For example, an adder is used to determine the address of the next instruction from the program counter, and a control ALU is utilized to compute the memory address of data to be operated upon by the CPU. With a working register established, it will be necessary to develop a way to utilize the register directly by furnishing it with operations. The development goal of designing a working ALU furthers the overarching goal of creating a working processor, as the individual components of the ALU are used in many places.

The first task of the ALU portion of this proposal is to demonstrate a simple half-adder circuit patterned in SiC. The half-adder will be fed alternating values of HIGH and LOW on both inputs to determine if a correct value is generated for the output and/or the carry of the circuit. The input for this circuit will be generated by an external microcontroller such as an Arduino prototyping board timed to send the signal pulses at regular intervals and check the answers. The Arduino will log the inputs, output, and carry from this circuit to check for errors. This experiment will be repeated in both standard conditions and harsh conditions in excess of 500 °C to simulate real operation. The Si based Arduino signal generation and logging can be isolated from the elevated temperature conditions since HIGH and LOW signals are not subject to the same trace length constraints as oscillators. Success of the circuit will be determined by continuous operation without errors with a lifetime to match previously explored logical units [30], discussed further in Section 5. The next task will be to scale up to an 8-bit full adder, then to a carry-lookahead adder which will be tested in the same manner.

Comparators and shifters will be tested in a similar “bottom-up” manner and constraints as the adder. For the comparator, initial tests will be performed on a simple 2-bit comparator, then scaled up to a full 8-bit comparator. The first bit shifter will be a simple array shifter of 2-bits, tested by cycling the bit shift by one unit over time. The scaled up version of the shifter will be tasked with performing full 8-bit shift operations. Tests will be performed at a range of temperature conditions and deemed successful for continuous operation without errors, similar to the other proposed units of the ALU.

4. Explanation of new or unusual techniques

The proposed work is unique in that it uses SiC for logical device fabrication. There are very few devices which use this technology, and use of substrate materials outside of Si in CMOS design is uncommon due to industry focus on small feature sizes and high computational power. While this is a new and exciting area of research, there will be few novel techniques implemented at this stage. The design flow for computers at the Si level is so well documented that many university level research institutions offer courses on the most up-to-date processes available. However, the technology for SiC devices is young compared to Si devices, many of these modern methods make use of architecture and design technology not prepared for the simplicity of SiC. Therefore, a “back-to-basics” approach is more suitable for this project. The “back-to-basics” approach will favor the use of the original VLSI work pioneered by Mead and Conway, and their contemporaries [31] [32] [33].

While much of the design for modern integrated circuits is handled by computational algorithms, few such mathematical models are available for the SiC substrate. The current state of circuit simulation makes use of NMOS SPICE models with certain parameters ignored [11]. Instead, a more traditional approach to device design must be considered. Masks for experimental devices will require manual generation based on traditional standard for VLSI device production pioneered by Mead and Conway [31]. Previous work by the SiC group at NASA has produced some important values for device modeling including size constraints. Physical constants observed in this work will be used to further develop a custom technology profile for SiC in a VLSI design program such as MAGIC [28]. By recording these parameters, it is possible to approach the algorithmic simplicity enjoyed by traditional substrate devices, establishing Electronic Design Automation (EDA). The modularity and regularity inherent to the nature of computer design makes these values extremely valuable as the overarching project scales up. Furthermore, VLSI EDA programs can be used in conjunction with SPICE to simulate the designed circuits at the pattern level [34].

5. Expected results and their significance and application

The qualifications for an individual circuit success were previously mentioned in Section 3. Generally, these success criteria can be divided into data retention time for register devices and continuous error-free operation time for ALU related devices. Some electronic properties are temperature dependent, such as resistance [19] and band-gap [35]. Therefore, it is critical to test circuits designed for optimal operation at one temperature over the range of possible operations temperatures. These tests are to be replicated at both standard temperatures and temperatures found in harsh conditions, such as the surface of Venus, using lab ovens.

A successful register is a device which can retain the stored data over many clock cycles, and change each bit in a timely manner on the rising edge of a single clock cycle. To determine the size of this clock cycle, we look to a reference point, such as a previous lander. The Sojourner Rover utilized a variant of the Intel 8085 chip, the 80C85, running at a clock speed of 2 MHz} [36]. The manufacturer reported maximum clock speed of this CPU is 5 MHz} [37]. Therefore, for the purposes of this test, the success factor will assuming an acceptable operation speed of 2 MHz} with an optimum operation speed of 5 MHz}. If we assume that the bit stored in the flip-flop is either modified on each clock cycle or stored for the clock cycle, and we assume that the chance for a bit to be either 0 or 1 is equal, then we can be 99% confident the longest a bit will be stored in a single flip-flop is 7 clock cycles. Therefore, the produced register will be adequately successful if it can retain a bit for 350 ns and exceed success if it can retain a bit for 1.4 μs.

A successful ALU is a device which can perform the requested operation quickly, with no errors. In a surface level lab with minimal radioactive sources, it should be expected that an adder or bit shifter would work consistently. However, no device is perfect, and chip manufacturers are hesitant to publish failure rates in their devices. Certain failure modes, such as electromigration, are tested for using temperature acceleration. However, the original draw of SiC circuits is the high temperature stability, reducing the likelihood of this failure path [38]. With the lack of benchmarks for ALU failure and the high temperature stability expectation which can accelerate further testing, the success metric will be based on the previous performance reported by the Hunter group, including a thousand hour lifetime of an oscillator at 500 °C [30]. The ALU produced in the course of the work in this proposal will be considered a success if the error-free lifetime exceeds 1000 h, the time reported by Spry et al.

In the event the devices fail to pass benchmarks for the register, ALU, or their preceding variants, efforts will be made to determine the cause of the issue. Previous circuit issues which have been a cause of concern in the past, such as the micropipe defects in SiC devices [38], have been analyzed in detail [39] based on “failed” experiments. This provides much needed data for future improvements on the material. Circuits which do not meet benchmark expectations will be analyzed by electron microscopic techniques for sources of damage, much like the previously mentioned failure was determined to be due to oxide cracking in the thousand hour test [30]. These sources of failure can provide more evidence to shed light on the cause of such defects in SiC systems.

The technology developed for this work has far-reaching applications. There have been few complicated logic devices made in SiC [30]. A successful register and ALU would open the doors to further development in this material, eventually leading to the eventual development of a single-cycle microprocessor CPU with high temperature and radiation resistance. Processors in this material would be very attractive for future space-faring missions and use in extreme terrestrial environments. The individual building blocks produced by this work can also be used as a basis for other SiC devices. For example, the development of a successful 8-bit register would naturally lead to the development of ROM and RAM with similar configurations. The modularity and repeatability of these devices makes it relatively simple to drop a successful gate design from one project to the next. By designing these devices in a prevalent, open-source VLSI EDA environment, the parameters explored in these designs would be available to future designers working with this technology.

6. References/Citations

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Giant Robber Fly Eating a Cicada

I took a lovely picture this summer of a giant robber fly eating a cicada. The picture was taken in Washington county, Oklahoma. I observed the take-down myself while working in the garage. The robber fly ambushed the cicada while it was sitting on an overhanging branch of the nearby hackberry tree.

Giant robber fly eating a cicada

In the above picture, the cicada is still alive. The panicked trills and beating of the wings allowed for me to easily find where the pair landed. The giant robber fly eating a cicada in this picture is of the genus Promachus. The species is difficult to know for certain without detailed observation of the specimen, but it appears to bear similar traits to Promachus hinei based on information available from BugGuide. Promachus hinei’s range extends through Oklahoma, making this a reasonable candidate. The yellow and black pattern, detailed in this picture, appears to be a distinctive trait.

Back from Hiatus

I have returned from the dead and am back from hiatus. This brief intermission was due to moving and real-life delays. Please enjoy this entertaining image of my cat carrying his favorite toy as a gesture, re-welcoming you to this blog now that I am back from hiatus.

He loves Da Bird

Book Binding – Part 2

The post last week showed you how to bind your class notes into a book.  This week, I will show how to finish compiling your documents with a hardcover bookbinding.  I chose to use a different example for my pictures here, as the bookbinding in these photos is more simplistic and instructive.

Materials

My hardcover bookbindings use matboard as the base material.  Matboard can be purchased from a hobby store which provides picture framing materials.  The board is cut to size, slightly larger than the paper which it is binding, with a razor and straightedge.

Next I use fabric scraps to prepare corners and spine coverings for the book.  For a more ornate book, I like to cover the entire matboard with one fabric, then use a second fabric for the corners and spine.  However, I want this post to show the bare minimum.  It is important to at least cover the corners of the matboard with material when binding a book in this manner, as the material is little more than thickened cardboard and can damage over time.

The corners are made by folding a trapezoidal shaped cutout of fabric at the corners.  The spine is made by using a strip of fabric to strap the matboard pices together.  Sometimes, I use a thicker material inside the spine fabric to improve the spine quality.

Cover to Book

The cover is secured to the book with at the edge of the spine.  A minimum, you can use a piece of gaffers tape to affix this point, but I chose to use colored paper.  The paper is glued to the the first page with a thin layer of PVA glue, and folded over.  The next half is glued to the hardened binding.

Note that the pictured example is NOT my best work.  This illustrates and experiment I made while coming up with this method.  The glue causes waves in the colored paper as it dries.  Furthermore, the cover which is thickened by the fabric causes waves as the paper settles into the matboard preferentially.  In later work, I have used another layer of material to thicken the matboard to match the fabric thickness.

Not the prettiest thing I've made, so learn from this mistake.

The end result is quite nice though.  The cover is substantial enough to protect the materials within, and the cloth hardcover bookbinding adds a bit of elegance to your shelf.  In my future experiments, I plan on adding embroidered titles to the spine before binding.

Hardcover bookbinding with cloth and matboard is a nice way to keep your documents on the shelf.