With regards to studying digital system design, hobbyists and college students have a whole lot of choices, whether or not it’s tinkering with field-programmable gate arrays or working up chip submission for a Tiny Tapeout run. However comparable instruments for analog design have been tougher to come back by. Actually you possibly can create analog circuits in a simulator like LTspice and take a look at them in opposition to concept. However nothing beats constructing actual analog circuits and measuring their precise, not theoretical, conduct. Measurement is difficult for analog designs as a result of the take a look at tools can have an effect on the very measurement being made.
To handle this academic hole, a staff led by me at Columbia College’s division {of electrical} engineering created MOSbius, a breadboard-friendly chip that you can imagine as a field-programmable transistor array for analog designs.
As its identify suggests, MOSbius is constructed round metal-oxide-semiconductor transistors, divided between n-type and p-type transistors. As these are the constructing blocks of most analog built-in circuits right this moment, designing with the MOSbius gives expertise that’s straight related to creating actual chips.
The MOSbius has 68 pins, of which 63 connect with both particular person transistors or frequent analog subcircuits akin to present mirrors and op-amp phases. Two different pins are used for optimistic and detrimental provide voltages.
The MOSbius chip [top left] comprises all of the transistors required for a lot of analog methods. Mounted on a breadboard through a printed circuit board [middle], discrete resistors and capacitors [bottom] are wired as much as present different wanted parts. A Raspberry Pi Pico [bottom right] is used to program the MOSbius.James Provost
The remaining three pins are used for programming a built-in swap matrix. You can create circuits with the MOSbius by connecting the pins with exterior wires. But it surely’s simpler to make use of the swap matrix. This will join any of the 65 transistor, subcircuit, and voltage pins to a number of of 10 inside buses. The matrix is programmed by clocking in a easy stream of 650 bits—one bit for every attainable connection between a bus and a pin.
We program the MOSbius’s matrix utilizing a Raspberry Pi Pico working a Python script that turns a easy JSON textual content file into the specified bitstream. If you’d like, although, you should utilize just about any 3.3-volt microcontroller that helps Python.
The MOSbius runs at 2.5 V. The printed circuit board used to attach the pins to a solderless 830-contact breadboard shifts the Pico’s 3.3 V down when programming the matrix. The PCB additionally contains bias and over-voltage-protection circuits.
How are built-in circuit parts completely different from discrete parts?
Because the MOSbius comprises solely transistors, circuit components like resistors and capacitors are wired up externally as through-hole parts inserted into the breadboard. You may assume that utilizing these exterior parts would create an imprecise approximation of the conduct of an actual analog IC, which has on-chip resistors and capacitors constructed out of silicon somewhat than, say, a steel movie or ceramic materials.
However at sign speeds under a few megahertz, the precise situation is that these exterior parts are overly exact. (Above 1 MHz, issues just like the stray capacitance of the breadboard start to turn into a problem.) Even an affordable through-hole resistor may have an precise resistance that’s inside 5 to 10 % of the worth it claims to be, and through-hole resistors with 1 % accuracy are frequent. Against this, built-in parts can simply range by 30 % from their nominal worth.
Consequently, actual analog circuits must be constructed with a substantial quantity of inside margin. Creating the ability to do that depends on understanding what’s occurring at completely different factors throughout the circuit, as enter alerts and part values range. And that requires making real-world measurements.
The “aha” second of seeing your circuit come to life will keep lengthy after the equations are forgotten.
In a simulated circuit, you possibly can click on on any node and skim out any parameter with out affecting the circuit’s conduct in any approach. With the MOSbius, you possibly can equally entry almost each node in a circuit, however now you face the fact that wiring up take a look at probes to make a measurement can change the circuit’s conduct. Determining probably the most elegant option to make measurements is essential for anybody hoping to see their concepts dedicated to silicon. The “aha” second of seeing your circuit come to life will keep lengthy after the equations and evaluation are forgotten
MOSbius’s capability to let of us poke round inside the heart of an built-in circuit design takes inspiration from the Three Fives Discrete Timer Package, which makes use of discrete parts to re-create the workings of some of the fashionable built-in circuits of all time, the 555. You’ll be able to in fact use the MOSbius to re-create a 555, in addition to many different demonstration circuits, beginning with the “Hiya world!” of {hardware}, a blinking LED.
Metallic-oxide transistors and generally used subcircuits are related to a lot of the pins of the MOSbius chip. They will also be wired collectively internally utilizing a set of buses programmed utilizing a 650-bit stream from a microcontroller. James Provost
The origin of the MOSbius was serendipitous: Whereas getting ready a set of scholars’ IC tasks to be fabricated, we realized we had room to squeeze yet one more chip into the batch if we may meet the transport deadline. Six weeks later, the MOSbius was off to manufacturing!
From this restricted run—150 chips in complete—we’re making MOSbius kits accessible at a nominal value of US $150 (topic to vary and simply to get well our prices) to different educators involved in bringing it to their labs. We’re additionally engaged on a revised model. The MOSbius can be utilized to make circuits that vary in complexity from these appropriate for undergraduates to graduate-level tasks. The Website online has an entire set of tutorials and a lab experiments guide able to go. In case you’re involved in acquiring a equipment, you possibly can contact me via the positioning.
I notice we now have solely a small variety of kits accessible—we’re in spite of everything a college division, not a semiconductor producer—however we need to make the MOSbius extra broadly accessible to each educators and fans sooner or later. The easiest way to make that occur is to contact me, as a result of sturdy demand is one thing we will current to a business accomplice!
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