On the subject of studying digital system design, hobbyists and college students have a number 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 return by. Definitely you may create analog circuits in a simulator like LTspice and check them towards 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 tools can have an effect on the very measurement being made.
To handle this academic hole, a workforce 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 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 built-in circuits immediately, designing with the MOSbius gives expertise that’s instantly related to creating actual chips.
The MOSbius has 68 pins, of which 63 connect with both particular person transistors or widespread analog subcircuits reminiscent of present mirrors and op-amp levels. Two different pins are used for optimistic and unfavourable provide voltages.
The MOSbius chip [top left] incorporates all of the transistors required for a lot of analog programs. Mounted on a breadboard by way of 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. Nevertheless it’s simpler to make use of the change matrix. This could 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 potential 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. In order for you, 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 consists of bias and over-voltage-protection circuits.
How are built-in circuit elements totally different from discrete elements?
Because the MOSbius incorporates solely transistors, circuit parts 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 moderately than, say, a metallic movie or ceramic materials.
However at sign speeds beneath a couple of megahertz, the precise concern is that these exterior elements are overly exact. (Above 1 MHz, issues just like the stray capacitance of the breadboard start to develop 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 widespread. Against this, built-in elements can simply fluctuate 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 totally different factors inside 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 may click on on any node and skim out any parameter with out affecting the circuit’s conduct in any means. With the MOSbius, you may 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 probably the most elegant solution 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 potential to let people 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 elements to re-create the workings of one of the well-liked built-in circuits of all time, the 555. You possibly can in fact use the MOSbius to re-create a 555, in addition to many different demonstration circuits, beginning with the “Hi there 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 making ready a set of scholars’ IC initiatives to be fabricated, we realized we had room to squeeze yet another chip into the batch if we might meet the transport deadline. Six weeks later, the MOSbius was off to manufacturing!
From this restricted run—150 chips in whole—we’re making MOSbius kits out there at a nominal worth of US $150 (topic to vary and simply to get well our prices) to different educators fascinated about 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 Website has an entire set of tutorials and a lab experiments handbook able to go. In case you’re fascinated about acquiring a package, you may contact me by way of the positioning.
I notice we’ve solely a small variety of kits out there—we’re in any case a college division, not a semiconductor producer—however we need to make the MOSbius extra broadly out there to each educators and fans sooner or later. The easiest way to make that occur is to contact me, as a result of robust demand is one thing we will current to a business companion!
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