Electronics for Newbies: A bit of an update

So, it's been a few weeks since my last post. As mentioned earlier, I'm now incrementally building a simple Z80-based computer on stripboard. It's been lots of little incremental things: Inspired by the pdf from here, I wrote a small PostScript file that generates chip labels for the chips I'm interesting in, which makes wiring everything up far nicer - no need to continually refer to the data sheet. I've also spent quite an extended period waiting for a cheap ZIF to arrive, which is perhaps slightly silly of me, rather than spending a couple of quid more for express delivery. Soldering a large number (or at least what feels like a large number) of wires is very tedious.

Initially I built and tested the banking circuitry. Having had experience with breaking things through shorts, and other surprises, plus having had enough experience of developing software, I'm taking a 'test early, test often' approach. After double-checking for shorts (the loupes I have from playing around with watches being fantastic for this), I'm now powering the circuit up with an ammeter attached, so I can make sure it's drawing the power I'm expecting. I guess somebody sufficiently paranoid would have some current-limiting circuitry in place, but that's outside my ability. Fortunately, nothing was shorting or otherwise drawing unexpected current.

The oscillator unit I'd plonked in, plus the banking circuitry, took around 35mA. Those 74LS-series little SRAMS are thirsty. I followed this up by soldering in the Z80 itself, and letting it run receiving NOPs from the banking software, and the power consumption's basically the same. Add in the EEPROM and the RAM with output enable lines high (i.e. off), current pretty much the same. Everything's noise compared to the power consumption of those LS chips. I think I'll really be going for pure CMOS in the future!

Data buses are fully wired-up, so all I need to do now is get the address bus in place. Then I can test my simple 'Cylon' program again, run some RAM tests, memory banking exercisers and I'll be ready for the next expansion: Adding a CTC and SIO. With timer interrupts and serial I/O, it'll be a proper little computer.

Posted 2013-09-14.