• Reflashing the BIOS

    Steven11/07/2021 at 18:35 0 comments

    Back in the old days EPROMs were used to store the BIOS. To erase them, you need UV light. I used a sunlamp for that, but I havn't got that one anymore. Besides the procedure to erase them was a bit time consuming, which increased the turnaround time of developing and testing. So I was looking for a better replacement.

    I found that the 27128 EPROM (16kB) could be replaced by a AT28C64(B) EEPROM (8kB), without changing the circuity.

    The tool chain I used in 1991 was MS-DOS based and ofcourse completely outdated. some parts were self written in turbo-pascal. To set up a new one, more future proof, I wanted to be abled to use it as well on windows as on MacOS, perferably open source. And it should be compatible with the old sources too. For now I am using:

    -vasm: assembler to assemble the 68k mnemonics to S19 (hex) file.

    -src_cat: convert the motorola S19 hex format to Intel hex, and to split the file for the two EPROMs (odd and even).

    -TL866plus: programmer & software to program the EEPROMs.

    ...and after thirty years, the new BIOS is flashed!

    This is the programmer my brother build to program the EPROMs, I wrote the software in turbo-pascal. Data was send by the parallel/printer port. In the box there should be a counter and latches, to program the chips. It's amazing what we build ourselfs those days...

    Now I am using this little box (TL866 plus) which is cheap to buy and very functional. Even the old EPROMs could be programmed and it is easilly connected by USB. The software runs on windows but I read it should also also run on linux under wine.

  • Reducing power

    Steven11/06/2021 at 18:16 0 comments

    Month ago I bought two CMOS replacements for the CPU: MC68HC000. The advantage of this CMOS CPU is that it consumes less power. Unfortunately the replacement was not that simple as you think. The replacement came from China and were used chips. They were desoldered and some of the pins were bent. But after some gentle wiggling I managed to get all the 64 pins in the right holes...

    The current in the system drops from 480 mA to 280 mA, so a reduction of 40%.

    Besides the power consumption and therefor a lower temperature, there are no advantages for this chip. In the old days I had a PC with the CMOS replacement of the INTEL 8080, the NEC V20. This CPU was 25% faster on the same clock speed as the original. I remembered that all the chips on the board were warm, but the CPU keeps cool.

  • Dusting the project

    Steven05/16/2021 at 14:16 0 comments

    I hadn't looked on to it for years (no time, creating an extra room in the attic and so on), but it kept itching... On the other hand it seems so strange fiddling around with old hardware, a simple ESP32 has more capabilities and speed than a 68k will ever have.

    Two reasons for dusting 1) I found out you could still buy very cheap (used) spare parts like 68000 chips (in China) and 2) I found this (and other sites) where many people greb their soldering iron and creating all kind of old and new -cool- hardware projects. And wouldn't it nice to combine this old hardware with new and very cheap hardware like SD-cards, RTC, 1-wire temp sensors and so on, like we use to do with Arduino and ESP32?

    The board was build in the ms-dos time, with turbo pascal and a cross assembler as main development tool  on a XT 8MHz IBM-clone (with by the way I overclocked it to 9 MHz). Since I went away from windows to MaxOs, I have to use (temperately) a virtual Box with 32 bits windows with the necessary tools. The goal is to use as many as possible standaards like VT100 (serial terminal program), Free pascal / Lazarus (as development tool), X-modem (uploading programs), FAT16 etc, so I won't be dependent from specific hardware/software.

    Clock circuit

    After powering up the board sometimes came alive but always was unstable. I found out one loose wire (for the interrupts) but that doesn't solve all the problems...

    As as student I didn't have an oscilloscope, in those days way to expensive. But this was the moment to buy one! For only about EUR 200 you can get yourself a reasonable one. After connecting it to the clock circuit of the board the problem became clear. On the scope you can see a sneaky spike after the first clock pulse, which let the CPU crash. And also the frequency is a strange 16.95 MHz (bottom left corner) with *every* kristal I used (from 2 till 24 MHz)?

    The clock circuit was copied from the one in the XT-pc; two inverters (with a 330 ohm resistor over every inverter) and the crystal between it. Only the circuit was mentioned for LS logic and I builded it with HCT (c-mos). The result is a very unreliable oscillator...

    So I rebuild it to a suitable circuit which worked with crystals from 5 till 24 MHz (ground tone only) using a 74HCT04 and after dividing it by two there is a stable and clean 50% duty cycle clock.

    Lessons learned: never modify a circuit unless you completely understand it fully.