Damn....My new 4028 chips arrived at the weekend (the BCD to Decimal ones), and I got all excited and rewired my breadboard prototype to try it out. Once it was all wired up, I switched it on and....bugger....didnt work.... I spent the next day or so trying to figure out why!
Here's which I think is going wrong... The old chips had a LOW TTL level output of around .6volts, where as these actually output 0volts. The problem being, that since I use pull up resisters to increase the brightness I need some sort of voltage there so that the power from the pullup doesn't power the LCD matrix when I dont want it to. But with 0volts, the pull up supplies power to both. Damn.
So, next thing to try is a few transistors, although Im almost at the point where I'm thinking sod the extra expense... get the slightly larger PIC (which will probably cost about 50-60pence more per unit, but save power, complexity and wiring - it almost has to be worth it.