In terms of studying digital system design, hobbyists and college students have quite a 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 related instruments for analog design have been more durable to come back by. Actually you possibly can create analog circuits in a simulator like LTspice and check them in opposition to principle. 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 check gear can have an effect on the very measurement being made.
To deal with this instructional hole, a team led by me at Columbia University’s department of electrical engineering created MOSbius, a breadboard-friendly chip that you can imagine as a field-programmable transistor array for analog designs.
As its title 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 integrated circuits at this time, designing with the MOSbius supplies expertise that’s immediately related to creating actual chips.
The MOSbius has 68 pins, of which 63 hook up with both particular person transistors or widespread analog subcircuits reminiscent of current mirrors and op-amp levels. Two different pins are used for constructive and destructive provide voltages.
The MOSbius chip [top left] incorporates 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 elements. 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 change matrix. You can create circuits with the MOSbius by connecting the pins with exterior wires. However it’s simpler to make use of the change matrix. This may 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 operating a Python script that turns a easy JSON text file into the desired 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 elements totally different from discrete elements?
Because the MOSbius incorporates solely transistors, circuit components like resistors and capacitors are wired up externally as through-hole elements inserted into the breadboard. You would possibly assume that utilizing these exterior elements would create an imprecise approximation of the conduct of an actual analog IC, which has on-chip resistors and capacitors constructed out of silicon slightly than, say, a metallic movie or ceramic materials.
However at sign speeds under a couple of megahertz, the precise challenge is that these exterior elements 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 could have an precise resistance that’s inside 5 to 10 p.c of the worth it claims to be, and through-hole resistors with 1 p.c accuracy are widespread. In contrast, built-in elements can simply fluctuate by 30 p.c from their nominal worth.
Consequently, actual analog circuits should be constructed with a substantial quantity of inside margin. Growing the talent to do that depends on understanding what’s happening at totally different factors throughout the circuit, as enter alerts and part values fluctuate. 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 browse out any parameter with out affecting the circuit’s conduct in any method. With the MOSbius, you possibly can equally entry practically each node in a circuit, however now you face the truth that wiring up check probes to make a measurement can change the circuit’s conduct. Determining essentially the most elegant option to make measurements is vital 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 potential to let of us poke round inside the heart of an built-in circuit design takes inspiration from the Three Fives Discrete Timer Kit, which makes use of discrete elements to re-create the workings of one of the vital common built-in circuits of all time, the 555. You possibly can after all use the MOSbius to re-create a 555, in addition to many different demonstration circuits, beginning with the “Whats up world!” of {hardware}, a blinking LED.
Metallic-oxide transistors and generally used subcircuits are linked to a lot of the pins of the MOSbius chip. They can 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 initiatives to be fabricated, we realized we had room to squeeze another chip into the batch if we might meet the delivery 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 focused on 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 initiatives. The Web site has an entire set of tutorials and a lab experiments manual able to go. If you happen to’re focused on acquiring a package, you possibly can contact me through the site.
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 wish to make the MOSbius extra broadly accessible to each educators and fans sooner or later. One of the simplest ways to make that occur is to contact me, as a result of robust demand is one thing we are able to current to a industrial associate!
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