Having now given up on CPC supplying the LED displays for the replacement circuit (I'm told they're due sometime in January), I've decided to look at reusing the original display. It's unusual as most LED displays these days are sealed units, where as this one has LED chips on a PCB with a white frame around that and a red diffuser on top.
Traditionally, you have two options to drive a 4x7 segment display, either directly drive each segment from an IO pin via a current limiting resistor, or multiplex the display digit by digit. The former option requires 4x7 = 28 IO pins and current limiting resistors and all segments are connected to ground. At 10mA per lit segment, it would use 280mA worse case. The later saves both pins and power, at the expense of more complicated software. The segments are all wired in parallel, but the grounds are switched per segment. So for example, although all the top bars (segment A) are driven from the same IO pin, only one of them will be switched to ground at any time. So the loop is:
- setup the output for the first digit,
- turn the first digit on (the display now shows 1_:__)
- wait a set time,
- then turn off digit 1
- setup for digit 2,
- turn on digit 2 (the display now shows _2:__)
- and so on
The display on this clock was sort of an unorthodox mix of the two. Rather than 1 phase for each of the 4 digits, this has only 2 phases, each of which controls bits of each digit. There are no current limiting resistors as the IO pins of the TMS3450 are current limited at 18mA. The common cathode drive transistors are also missing as it makes use or a rather neat trick, but one as I said, I don't think anyone would come up with these days. What it does is use the ac from the transformer which already alternates at 50Hz, fed via diodes, so that on each cycle of the mains, one of the common cathodes sinks current as it's on the negative cycle. The other is on the positive cycle, so the diodes are reverse biased and therefore off. The flickering of the F segment on the hours I used to see occasionally must have been due the timing drifting slightly out and this being set before the phase cycle was complete.
I've slowed down the cycle. Here is phase 1: