2007...: RC1802 CPU
First proto of RC1802 playing säkkijärven polkka
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2009-12-26: Time flies - been a busy year, not much done here. Did start floppy controller board for RC1802 based on D765(8272). New page opened for it.
2008-01-17: Tape output also works fine now. Not that much tested, but it can read original Nano tape, save it to another, and read back again without errors. Pretty much everything is now tested briefly, except the expansion bus. So it looks the PCB errata includes wrong symbol for the opamp and need to invert the serial lines, otherwise it looks working. Getting ready to release the schematics!
Also noticed one SW problem I need to solve - RCMON/1 tries to detect memory size during boot. It does it by writing to each page and then reading it back. It works, but it also overwrites anything in the memory. So everytime you press RST you loose the programs in RAM (first byte of each page). Have to fix this somehow.
2008-01-14: Tape input works! Just loaded Oscom Nano program from original tape and run it as is. Got the code loaded easily, but for some reason it didn't run first. After some disassembly I noticed, that the code assumes 4k RAM with no high address bit decoding as in Nano. It loads 5050h to index register, when it actually should be 0050h. This way it saves a byte of code I guess. RC1802 had 32k chip, so it loaded garbage. When I loaded the program twice, first to 0000 and then to 5000 it started working fine. Might also work, if I use 2k RAM chip.
Tape output also works electronically, but there is something to be fixed still in code. Looks like the tape circuit (equivivalent to ELF II Giant board) needs quite a high input signal level. Headphone output from Creative X-Fi soundcard don't give enough, needs a bit more. Made a small amplifier using LM386 and now my old tape deck with line level output works fine as does the X-Fi line out. Need to find some old tape recorder to test it really works without external help.
Looks like I need to make Nano hex-keypad and 1864 (1861 PAL version) video to run Nano games on RC1802!
If someone has audio files from COSMAC VIP, ELF II or similar, I'd like to try to load to RC1802 to see it also works there (Nano should use the same format). If you do, please send mail, address in page footer.
2007-12-27: New year approaching - IDIOT/4 _does_ work, but the serial level shifter circuit does not allow echo on bit level (it steals negative voltage from RX-line to TX and gets invalid signal with full duplex). So if local echo is not used, it does work now.
RAM tested with 6116, 6264 and 62256. RCMON/1, my monitor program, also looks to be useful. Tape interface is under work next. Test code does output signal, that looks correct (uses same format as COSMAC VIP & others), but tape input is yet to be tested. FFT analysis from saved 2kHz header does not look clean, propably duty cycle of the square wave is not 50%. I wonder is it in Giant board or VIP? (anyone have those to test??)
Started preparing user's manual as well.
2007-09-13: Serial port is now inverted using the opamp, now it works with old code also (like Giant Board). Still original IDIOT/4 binary did not start, needs more study.
2007-09-11: Serial port also looks working. I tried to use IDIOT/4 monitor code, but it looks I missed the fact, that serial output is kind of inverted there. I mean Q-output is low on idle, whereas I implemented the serial driver circuit to expect Q high on idle (as "normal" serial port does). So it still works fine, but I had to modify the code. There are two op-amps unused on the LM3900, so I could easily invert the serial output. Maybe I will to make it more compatible.
RAM works at least with 62256, have to test the smaller chips as well.
Oops - I have used incorrect PCB symbol for the LM3900, pinout is incorrect => need to make some soldering to fix it.
2007-09-02: and it works! Not everything tested yet, but it did play säkkijärven polkka (EEPROM borrowed from K-Elf). First it looked dead, but I had to replace 4071 with a 74HC32 (That's the soldered wire stuff beside the crystal). They have different pinout and I made a mistake there first. After fixing it looks working. Next step is to test serial port, RAM and tape I/O.
2007-09-01: Proto PCBs arrived yesterday! assembling...
RC1802 ("Retro CPU 1802") is finally completed to prototype.
It came out as a hybrid of several ideas I had earlier. CPU board is 160x100mm:
It includes
- CDP1802 running at 2MHz (or lower by changing the crystal)
- 28-pin socket for ROM. This can use 2716,2732,2764,27128,27256 (EPROM) or 2816,2817,2864(EEPROM) for 2k-32k ROM
- 28-pin socket for RAM. This can use 6116,6264,62256 for 2k-32k RAM
- Boot circuit to allow automatic boot from ROM, but later map RAM to 0000h-7FFFh
- Glue logic made with standard logic circuits (most 4000 CMOS)
- Serial port (uses Q and EF2 or EF4). This is SW compatible to ELF II giant board
- Tape recorder interface. This is also compatible to ELF II giant board or COSMAC VIP
- Expansion connector, 2x25 pin header (for video,disk,keypad,...). All signals are routed here
I also call this "Tel-Elf" as with an expansion board for keypad and video it could run most Telmac or Elf SW. This is more or less just a CPU board. The only I/O is uses is OUT4 (64) to reset the boot flag. It is fully decoded, i.e. decodes all 3 N-lines. Memory mapping is only based on highest address line. If smaller than 32k chips are used, the memory can be externally extended using properly DSBL-signal, that disabled all on-board memory. External memory can also overlap the ROM.
Board needs external power regulator as it uses 5V DC. I added a 5V6 zener and a fuse just in case I by mistake connect higher voltage or reverse the polarity.
Let's see how well this works. When it works fine I'll design video/keyboard ("RV1802") and floppy interface ("RD1802"). Hard drive could be easily added, but to me it feels too modern. 5.25" floppy would do fine. Video is still somewhat open, but it definately needs at least 1861 compatible video.
When this is quickly tested I'll publish the schematics here as well.
