This circuit, Fig. 16L.1, the most fundamental of flip-flop or memory circuits, can be built with either NANDs or NORs. We will build the NAND form. It is called an SR flip-flop or latch because it can be “Set” or “Reset.” In the NAND form it also is called a “cross-coupled NAND latch.”

To this point we have treated positive feedback as a bad thing – as a result that you get when your pen slips, interchanging senses of feedback.

The power used in a CMOS circuit is directly proportional to clock frequency. Current only flows into or out of a CMOS gate to charge or discharge the input gate-to-source capacitance when the input switches from low to high or visa-versa.

Suppose we want to measure the width of a short, positive-going pulse that occurs infrequently using the hardware shown in Fig. 25W.1.

To do all of today’s lab is a challenge: the op-amp circuit is the most complex that you’ve built so far, and if some stage holds you up, you’re likely to run out of time. But that shouldn’t worry you. Only the differential amp (§5L.1) – not its conversion into an op-amp – is fundamental.

We have advertised the differential amp as just a pair of common-emitter amplifiers, and have promised you that there’s not much new to understand here: you can use what you know from the two earlier labs where you build C–E amps. But students have noticed some effects that are new: not what one might expect from experience with a single C–E amp.

Today we look first at one more benign use of positive feedback, an active filter, and then spend most of our time with circuits that oscillate when they should not. In this lab, of course, they “should,” in the sense that we want you to see and believe in the problem of unwanted oscillations. On an ordinary day, the oscillations that these circuits can produce would be undesirable, and would call for a remedy. Some of you have met these so-called “parasitic oscillations” in earlier labs.

A student asked a good, hard question, recently. I was stumped, till the answer struck me – more or less the way the apple is said to have bonked Newton on the head – next morning as I cycled to work.

The “Cortex Debug/Program Connector” is a standardized hardware interface used both to program the internal flash memory and to debug program code by single-stepping program instructions and reading and writing the internal registers of the CPU.

In the previous chapter we saw that positive feedback can be useful – though it is the underdog in feedback circuitry, less important than the great strategy of negative feedback. It can make a switching circuit decisive, and it allows construction of oscillators.

In this chapter we will begin with an introduction to interrupts, then learn how to configure I/O pins for input, first to read a pushbutton and then to scan a matrix keyboard. We will also look at creating Finite State Machines in the microcontroller.

We are going to configure the SERCOM with the simplest of the three protocols, SPI, requiring only a clock and data line along with an optional device select line.