| new, small footprint 68000 [message #10243] |
Tue, 14 February 2023 12:17  |
norwestrzh
Messages: 196
Registered: November 2015
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Senior Member |
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Here's a new, small 68k SBC. It has 2MB of SRAM, 128k EEPROM, a 68681 DUART (2 serial ports @ 38,400) and a compact flash interface. The CPU I'm using is an MC68HC000FN20 that runs at 25 MHz. I got a couple of HC Coldfire CPUs (from Germany, I think) that also run at 25 MHz. Everything on the board is either PLCC or through-hole 74LS. The Winbond W27C512 EEPROMS are the only programmable components. It's an excellent little 68k platform that can be easily tinkered with. I've written a little utility to exercise the compact flash, and I can read and write sectors to the CF, and get it to cough up its ID data. Working on a CBIOS to run CP/M68k at the moment.
Please forgive the bodge for the MCP130 watchdog. When I was working on bringing up the board, I was getting spurious resets/halts so I thought that it might be causing them. I removed it, and later found that it was working just fine. At that point, with the other stuff soldered onto the board, I found it very difficult to try to re-mount the thing. So it is patched onto the board. The next version that I build will be much more photogenic!
I'm wondering if it would be possible to run fuzix/mCLinux on it?
Many thanks to Plasmo for his help with some of my bone-headed mistakes, and for offering help with things 68k!
Roger
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| Re: new, small footprint 68000 [message #10783 is a reply to message #10247] |
Sun, 26 May 2024 14:16  |
norwestrzh
Messages: 196
Registered: November 2015
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Senior Member |
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Here's a new variant of the small 68k described above. This one uses a MC68SEC000FU20 CPU. I find soldering TQFP 0.8mm chips very challenging, but my friend Johan soldered one on an adapter PCB for me. He's a soldering whiz!! I just replaced the PLCC 68000 with four 8x2 headers to accommodate the adapter board. Otherwise, the design is essentially the same. Johan told me that he was able to "free run" that CPU at 50 MHz. I find it will run in this board at 32 MHz, and it is very stable. I tried it at 40 MHz. The monitor will run OK, but it won't boot up CP/M 68k. I think that 40 MHz is right at the limits of the memory and support circuitry. I'd like to try it at 36 MHz, or maybe 38, but I don't have the oscillators to do it. I'll have to order some.
I think it is amazing that a simple through hole, 2 layer PCB can run at those clocks.
Roger
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| Re: new, small footprint 68000 [message #10802 is a reply to message #10801] |
Tue, 25 June 2024 12:47 |
plasmo
Messages: 916
Registered: March 2017
Location: New Mexico, USA
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Senior Member |
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By "mezzanine board", floobydust may mean a plug-in board for EPM240 development board. In fact, I've designed such a 68SEC000 mezzanine board for EPM240. It has 68SEC000, 2x 512K RAM, SD card, serial port, and RTC module. It has not been not prototyped, I tried to see how well it fit with the EPM240 dev board. Ideally I want to use SDRAM instead of two 512K SRAM, but it is better to take baby steps. I plan to use the 1KB embedded flash to bootstrap 68K monitor stored in SD card. I don't have microdrive, so I'm more confident with SD card bootstrap or CF disk bootstrap.
Bill
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| Re: new, small footprint 68000 [message #10803 is a reply to message #10802] |
Tue, 25 June 2024 13:46 |
floobydust
Messages: 9
Registered: September 2020
Location: Boynton Beach, FL
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Junior Member |
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Roger, Bill,
I guess I wasn't completely clear... the Microdrive I'm referring to is quite a bit smaller than a Compact Flash card (without the CF socket). The interface is on a 35-pin 0.3mil spacing FPC connector and despite the pinout being 35 pins, it is a full IDE interface. I've been using these for a few years now and very easy to support, as any IDE BIOS or code should work without any modifications.
I'd be happy to send Bill a drive with the FPC cable and the SMT socket to see if he can work it in somehow.
I also have the full programming manual which also includes the full electrical and mechanical specifications.
I've also been looking at SMT SRAM, which can be found in 16-bit parallel single package in larger sizes as well, which could reduce the overall physical RAM space quite a bit, but I'm certain you know about these as well.
Regards, KM
https://github.com/floobydust
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| Re: new, small footprint 68000 [message #10804 is a reply to message #10783] |
Thu, 27 June 2024 10:45 |
jbforrer
Messages: 25
Registered: May 2017
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Junior Member |
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Greetings,
I built up one of these cool 68SEC00 boards that Roger kindly donated.
The CPU has a 20MHz marking but it free-runs fine at 50MHz.
Experimented with a 50MHz oscillator installed on the board, amazed that it actually runs the monitor code.
That require CPU, DUART, ROM, SRAM and support logic to function properly at that clock rate, basically with DTACK grounded (no wait states).
I've not studied actual timing margins, just been changing clock modules. ROM is 45ns, SRAM 10ns, and the DUART is a Philips SCN68681.
ASM and C (GCC cross compiler) test programs, including memory diagnostics and ASCIIART benchmark loads and execute just fine. The S-record loader runs without flow control.
ASCIIART executes in 6.07s.
Presently, the 82C55 only works for me up to 28.3MHz. So that is what CP/M68K runs at. Not bad either. It would require a small change to add wait states for the 82C55.
Thanks for sharing this project Roger.
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