Mind to Mind

The Power of Pullups

Hacking away on the PIC16F88 trying to get it to talk to the NeXT host machine, and I kept getting very long rise or fall times for the signal when twiddling the TRIS bits. I also saw some very unusual signals on the clock line. It finally occurred to me that as an open collector bus, with no keyboard attached, the lines would float without the addition of pullups. I added two 10k resistors, and lo and behold, deterministic timings! With that I was finally able to start sending data to the NeXT machine.

I never claimed to be an electrical engineer… but even so.