Wow, I hope this does not defend anyone, but this seems to have gotten complicated beyond reason. Why are we even talking about bit banging and interrupts?

Read a character from memory, send it out the UART. Correct me if I'm wrong, but this is the task, right?

If so, use a non-WDC 65C51, and use ...

...There are actually three choices under consideration for this application:
Use a working UART.
Use bit-banging.
Use a W65C51, which cannot tell you when you may write another byte to the transmit register.

In my opinion, this is in order of easiest to most difficult for this particular ...

I do something like
STZ ACIA_STAT ; Reset ACIA by storing 0 in its status register.

LDA #00011110B ; Set for 1 stop bit, 8 data bits, 9600 bps by
STA ACIA_CTRL ; storing the number in the control register.

LDA #00001001B ; No parity or rcv echo, RTS true, receive IRQ but no
STA ACIA_COMM ...

You could port the app. Its not that complex. I had a quick go this evening.

https://youtu.be/X6lun7d0JLI

I tried porting the C but there is a problem in wdc02cc that makes it crash (probably a pointer issue in the compiler itself -- I tested it on Windows 10 and XP in the virtual machine).

In ...

Systems thinking is a good skill, and a UART is a good level of system component.
Indeed. If memory correctly serves me, a single-chip UART was on the market in 1970 or 1971 to act as a basic system interface device.

In making the decision to bit-bang or go with a UART, I think the question to be ...

I still like the idea of using LZ4 compression. I tested with output from the sl command (which I haven't come across before, but I'm now going to install it everywhere I can), and LZ4 compressed ~49kB down to 8926 bytes. Sooo, it should be possible to implement this with a 65(c)02, a 32kB eprom ...

One might try for least hardware, or for easiest project, or for maximum educational value.

I'm not sure, in fact, whether a system with a UART doesn't have more educational value than a system which bit-bangs. Systems thinking is a good skill, and a UART is a good level of system component. A ...

I have seen the schmitt triggered version of the NAND gates used in many other designs and thought that they were appropriate here, thinking that perhaps my solderless breadboard build may be a bit more noisy on the clock line and that those would compensate better. Would 74x00's be better for this ...

Many thanks to all of the great responses I got back on this yesterday. There were a lot of problems with the schematics from round one, everything from not qualifying the CE's and OE's properly, to not even including enough ROM chips for what I mapped out. Yikes! :oops:

But thanks in no small ...

Exactly. I will be putting the commented assembly code in the case too so that would be a serious buzz kill to see it just calling sl to run on the hidden pi. Need a little more code than that to show.

I’ll also be putting the schematic in the case as well, broken up into sections that make sense ...

The purpose of the device is to produce some sort of output that may be eye catching enough to generate questions and interest. It will sit in a glass display case in the hallway in our ECE department where it will be seen mostly by ECE and CS students. A demographic that in theory should have an ...

No. I’ve captured the ascii sequence produced by sl and that is the data array I’m storing in ROM. Just calling sl on the pi is too simple. I want the 6502 to do a little more than that. The only reason the pi exists is because we don’t have any old serial terminals left on campus. The only one I ...

I don’t think I’m going to go the compression route. I’m confident I can get the two rom chips working. More confident in that than I am with what is going to be much more complicated code on an unfamiliar platform. I think I need more practice with the idioms used in these 8 bit machines before I ...

floobydust
Well, I already implemented a delay timing workaround when I discovered the Xmit bug some years ago. You can reference it here:

viewtopic.php?f=4&t=2543&start=30#p29795

Just change a couple parameters for the baud rate and CPU clock rate and you're good to go.

oooh that looks pretty ...

BigEd, yeah I like that countdown method better. I'm just going to have to get used to working with high byte and low byte, whereas with CortexM I really never needed to worry about that unless I was assembling bytes into larger datatypes I got from I2C devices or UART.

I have access to 200MHz ...