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How to use a 6801/6803 with external ROM in place of 6801U4 (with internal ROM)

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Hi all,
I'm dealing with a vintage MC6801U4 MCU that I need to program (I have the internal code) but I'm unable to do it since I don't have a proper programmer (very few programmers can handle this device).Therefore, I was wondering if I can use a MC6801/6803 in expanded mutiplexed mode 2 (internal RAM and no internal ROM).Datasheet of MC6801/6803 gives some hints on how to design the circuit so I made an attempt but it does not work.I attach my schematic and datasheets of MC6801U4 and MC6801/6803 hoping someone will help me.Thanks in advance.

First, you cannot program the MC6801U4. It has a mask ROM.

But you can program the MC68701U4, which has EPROM (windowed DIP-40).

This application note describes how to let it self-program:

Careful! Always cover the chip window unless erasing (ask me how I know!). These MCUs are incredible sensitive to light, as opposed to standard 2732 EPROMs.


--- Quote from: Benta on June 14, 2024, 11:26:11 pm ---First, you cannot program the MC6801U4. It has a mask ROM.

--- End quote ---

Yes, sorry.

--- Quote from: Benta on June 14, 2024, 11:26:11 pm ---But you can program the MC68701U4, which has EPROM (windowed DIP-40).

--- End quote ---

Yes, I know, but very few (expensive) programmers can do it and, anyway, I'm not interested in programmig the MC67701U4.As I said, I was thinking to replace these MCUs (6801U4 or 68701U4 with a regular 6801/6803 interfaced to an external EPROM that store the internal code of the aforementioned MCUs.


Your circuit should work but check the worst-case timing of the data reads from the EPROM. Depending on the processor clock speed and EPROM speed rating the data read from the EPROM might not met the set-up time for a processor data bus read.  You could dispense with the /MEM_RD input to the 74138 decoder to eliminate the extra gate delay from IC5A and gain a little bit extra timing margin.

You should check that the mode setting inputs are not used by the S/W as normal I/O since you have hard-wired these to Vcc and Gnd.

Similarly, you need to check that I/O ports P30-37 and P40-47 are not used in the program. If the 6801U4 was used in the single chip mode then you would normally expect these I/O lines to be used. Since these lines are lost when connecting an external EPROM they would need to be replaced by external logic. The 68HC11 family had a chip specifically for this purpose, the 68HC24 port replacement unit, but I don't think the 6801 family had an equivalent chip.

There were also micros in that era with a piggyback ROM : a socket mounted on top of the processor. I have one somewhere : I forget which actual device it was but I think it was 6800 family.

So the micro was effectively in a bigger package, but it retained the DIP footprint of the masked package and added further pins on the top surface for a ROM data bus.


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