On the subject of studying digital system design, hobbyists and college students have lots 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’ll be able to create analog circuits in a simulator like LTspice and take a look at them in opposition to principle. However nothing beats constructing actual analog circuits and measuring their precise, not theoretical, habits. Measurement is difficult for analog designs as a result of the take a look at gear can have an effect on the very measurement being made.
To deal with this academic 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 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 integrated circuits right now, designing with the MOSbius gives expertise that’s immediately related to creating actual chips.
The MOSbius has 68 pins, of which 63 connect with both particular person transistors or widespread analog subcircuits similar to current mirrors and op-amp levels. Two different pins are used for optimistic and adverse provide voltages.
The MOSbius chip [top left] accommodates all of the transistors required for a lot of analog techniques. 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 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 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 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 text file into the desired bitstream. If you would like, although, you need to use 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 totally different from discrete parts?
Because the MOSbius accommodates 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 habits of an actual analog IC, which has on-chip resistors and capacitors constructed out of silicon reasonably than, say, a steel movie or ceramic materials.
However at sign speeds beneath 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 out to be a problem.) Even an inexpensive 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. Against this, built-in parts can simply fluctuate by 30 p.c from their nominal worth.
Consequently, actual analog circuits must be constructed with a substantial quantity of inside margin. Creating the talent to do that depends on understanding what’s happening at totally different factors throughout the circuit, as enter indicators 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’ll be able to click on on any node and browse out any parameter with out affecting the circuit’s habits in any approach. With the MOSbius, you’ll be able to 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 habits. Determining essentially the most elegant technique 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 means to let people 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 parts to re-create the workings of probably the most common 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 many 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 tasks 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 obtainable at a nominal value of US $150 (topic to alter and simply to recuperate 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 tasks. The Web site has a whole set of tutorials and a lab experiments manual able to go. When you’re focused on acquiring a package, you’ll be able to contact me through the site.
I notice we now have solely a small variety of kits obtainable—we’re in any case a college division, not a semiconductor producer—however we wish to make the MOSbius extra broadly obtainable to each educators and lovers 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 will current to a industrial associate!
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