Relating to 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. Definitely you possibly can create analog circuits in a simulator like LTspice and take a look at them towards concept. 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 tools can have an effect on the very measurement being made.
To handle this instructional 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 at the moment, designing with the MOSbius gives expertise that’s instantly related to creating actual chips.
The MOSbius has 68 pins, of which 63 hook up with both particular person transistors or widespread analog subcircuits resembling present mirrors and op-amp levels. Two different pins are used for constructive and detrimental provide voltages.
The MOSbius chip [top left] accommodates all of the transistors required for a lot of analog techniques. 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. 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 inner 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 operating a Python script that turns a easy JSON textual content file into the specified bitstream. If you’d 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 elements totally different from discrete elements?
Because the MOSbius accommodates solely transistors, circuit parts like resistors and capacitors are wired up externally as through-hole elements inserted into the breadboard. You may assume that utilizing these exterior elements would create an imprecise approximation of the habits of an actual analog IC, which has on-chip resistors and capacitors constructed out of silicon quite than, say, a metallic movie or ceramic materials.
However at sign speeds beneath a couple of megahertz, the precise subject 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 inexpensive 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. In contrast, built-in elements can simply fluctuate by 30 % from their nominal worth.
Consequently, actual analog circuits need to be constructed with a substantial quantity of inner margin. Creating the talent 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 possibly can click on on any node and skim out any parameter with out affecting the circuit’s habits in any approach. With the MOSbius, you possibly can equally entry practically 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 probably the most elegant strategy 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 center 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 vital well-liked 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 “Hiya world!” of {hardware}, a blinking LED.
Steel-oxide transistors and generally used subcircuits are linked to many 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 making ready a set of scholars’ IC tasks to be fabricated, we realized we had room to squeeze yet another chip into the batch if we may meet the delivery deadline. Six weeks later, the MOSbius was off to manufacturing!
From this restricted run—150 chips in whole—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 all in favour of 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 page has an entire set of tutorials and a lab experiments guide able to go. In case you’re all in favour of acquiring a package, you possibly can contact me by way of the location.
I understand 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. 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 business accomplice!
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