This time, I'm attempting to create a system using the 6809 microprocessor. It's one successor to the original 6800 architecture, but adds lots of 16 bit features, such as two 16 bit index registers, another 16 bit stack pointer, and the ability to combine the two accumulators into one 16 bit register. These characteristic make the 6809 very versatile.

Part list

• MC6809 (NMOS, 4 MHz)

Now, I don't have much to show you yet, but I always start these projects by getting something rather simple to work. The first task was to write output data to a Peripheral Interface Adapter (PIA). However, oddly enough (and after verifying every connection and details of the datasheets), the output to the PIA were... sporadic. Unlike the CMOS versions of the 6502 and Z80, the 6809 is not a static design, and you therefore cannot halt the clock to single-step instructions and monitor the bus, which I used to do with a very ad-hoc Arduino setup.

After a lot of headscratching, I decided to buy a cheap logic analyser with a resolution up to 24 MHz and record it with Sigrok. I inspected the address lines, data lines, and some control lines. It turns out that everything ran perfectly for maybe a millisecond. Inspecting the E (enable) line, which is the last signal in the picture, everything starts going wrong after it pulses a tiny bit shorter than usual. This made me realise that I accidentally used ever so slightly larger load capacitors in the crystal circuit than spec (27 pF vs. 24 pF). Even single-stepping wouldn't have caught this.

Just as an end note, after getting the basic microprocessor setup to work on the 4 MHz crystal, there was something more odd that I noticed with this system. I turned off the lamp on my desk, in order to get ready to leave, but the output of the PIA changed. Somehow, somewhere, signal noise or a current surge generated from the lamp indirectly channels into the PIA to change its state. How peculiar.