The bad capacitor makes sense. It's easy to damage MLCCs or monolithic ceramic leaded capacitors when soldering. I mentioned the problem with no capacitor (which would include an open capacitor) in the I/O ICs section of the 6502 primer, but there's a hand-drawn diagram of what I got on the oscilloscope in the 6th workbench computer QRG page scan at http://wilsonminesco.com/BenchCPU/B1QRG/ under the text "ACIA1 crystal circuit:". Even though the 1.8432MHz signal was there, there was some other stuff on it that kept the ACIA from working when the capacitor was not there.


XTALI and XTALO make sense if you think of "I" and "O" being short for "input" and "output."

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Think of pin XTLI as an input to a sensitive amplifier, and XTLO as the output from this amplifier. When you put your 'scope probe on XTLI, the capacitance and resistance of the probe, lead and input of the 'scope are enough to degrade the signal. The input to the amplifier is then outside the correct range, so the circuit stops oscillating.

Whereas, when you test on XTLO, there is still enough drive so the circuit continues to oscillate.

Nearly all crystal oscillator circuits need either one, or sometimes two loading capacitors to help the crystal to oscillate in the correct manner.

Mark

Thanks all for your help. Things are getting a bit clearer now. After tinkering with it some more for an evening, I managed to reliably send a string of text to my terminal.

The only issue i'm still seeing is that when I immediately begin sending when the power comes on, I receive a bunch of crap data.

If I press my reset button, it's always fine. I'm using a DS1233 "econoreset". I'm considering just killing time for a few milliseconds to get things stable on boot.

As you had earlier problems with the baud clock oscillator not starting, it's possible that it's slow to start or (slow to) become stable. I would suggest using a delay loop in your startup routine that's called before sending any data to the 65C51. This will give the chip some time to settle before trying to transmit characters.

_________________
Regards, KM
https://github.com/floobydust