The next “milestone” for the 6809-on-a-breadboard computer project is to gain some RAM. Computers can’t do very much unless they can store things.
This was fairly easy, though I did have to fix one problem with the current circuit. I need to do some more research on this subject to understand the issue fully, but briefly…
Previously, in the EEPROM only test, I was feeding the E (Enable) output from the 6809 through to the Enable pin on the address decoder, which was then used to form the Chip Select for the EEPROM. Meanwhile the /OE and R/W line for the EEPROM came more or less directly from the R/W line on the 6809. This works ok (but wrong) when only the EEPROM is in play, but is not sufficient or correct when RAM is added.
The RAM is at the right.
To exercise the RAM, and prove that it is working correctly, I have made some changes to my test program:
- It now makes use of a couple of simple subroutines (which involves the use of a stack in RAM)
- The pattern of LEDs to show is copied from EEPROM to RAM at startup, and then shown from RAM
- The pattern is modified (inverted) after it is shown, so the next pass through the loop will show a reversed pattern
- The delay (speed) that the pattern is shown is held in a memory location and adjusted on each run through
It’s getting crowded in there! Right of the MPU (middle row) is the 4 in 2 address decoder, NOT gates (74HC04) and NAND gates (74HC00). The RAM is at the bottom middle. Left of it is the unwired MC6850 UART, which will be my next addition. That IC is the oldest on the board, dated the 18th week of 1982. It shall be interesting to see if it still works (I would be amazed if it did not).
Today at work I received the PCB for the EEPROM programmer. Total time to have the board made up in China and receive the board – less then 2 weeks. Pretty impressive stuff. Total cost, of 10 identical 10cm by 10cm boards, was about 37 pounds. Hopefully I will get a chance to solder up the board over the weekend. Whilst I’m happy with my stripboard programmer, the PCB version certainly looks a lot nicer.
Thanks again to Richard Gellman for turning my circuit into a PCB design!