#z85c30

The SCC is a 2-channel USART perhaps most commonly known for its use in early Macintosh computers for serial and LocalTalk. It’s not exactly the easiest chip to use though, with sixteen registers per channel which have to be programmed in the right order to get the thing operational. The manual calls attention to this, but then fails to specify what order they should be initialized. I had a heck of a time trying to get it to work the first time I used it, so I just copied what worked that time, and changed the appropriate registers for baud rate and clock settings. 

The hard part this time was just getting a stable enough clock. The SCC can generate a clock with a crystal between its sync and clock in pins, but on a breadboard it can be hard to get a stable clock that way. With modern hardware, I could receive whatever the SCC sent with no problems, but the SCC is not as flexible in its timing. It had about 1 in 20 odds of mangling a byte it was receiving. In the end, I replaced the crystal with a 1.8432MHz oscillator, and receiving data has been much more reliable. That particular clock frequency was chosen because it’s one of the standard clocks that can generate common baud rates without jitter.

As for the monitor program, I want something that can accept data from serial in Intel Hex format and copy it to RAM, then jump execution to any point in memory. I also want to be able to dump the contents of memory back out the serial port in Hex format. I’m sure I could find a monitor program and modify it to work with my setup, but where’s the fun in that?

Instead, I’m modifying the sketch I wrote to program my flash chips using an Arduino Mega, since it already has the ability to input/output Hex format over serial. That was certainly an interesting exercise—porting C++ code heavily dependent upon specialized libraries for a modern embedded microcontroller to C code for for a 1970s microprocessor. I’ve finished with porting it, and at the very least my menu structure is working, but I’ve got a lot of debugging to do.

Most notably, the system reboots nearly every time I call atoi() (but of course, not every time). I’m going to have to either dig deep into the standard libraries for SDCC or just write my own atoi() function.