These days, we take it for granted that you can connect a cheap piece of hardware to a microcontroller and have an amazing debugging experience. Stop the program. Examine memory and registers. You can see and usually change anything. There are only a handful of ways this is done on modern CPUs, and they all vary only by detail. But this wasn’t always the case. Getting that kind of view to an actual running system was an expensive proposition.

Today, you typically have some serial interface, often JTAG, and enough hardware in the IC to communicate with a host computer to reveal and change internal state, set breakpoints, and the rest. But that wasn’t always easy. In the bad old days, transistors were large and die were small. You couldn’t afford to add little debugging pins to each processor you produced.

This led to some very interesting workarounds. Of course, you could always run simulators on a larger computer. But that might not work in real time, and almost certainly didn’t have all the external things you wanted to connect to, unless you also simulated them. Continue reading “Lost Techniques: Bond-out CPUs And In Circuit Emulation” →