Dr Jefyll wrote:
Hi, Atlantis. Congrats on getting EhBasic running!
As for the Flash problem, the behavior you describe could be caused by improper chip-select to pin 7 of the Compact Flash card, resulting in it being active at the same time ROM or RAM is active, and thus interfering. What sort of address decoder have you provided to generate the chip-select? And do you have a 'scope or logic probe with which to observe the chip-select signal?
Im my design there is 8kB of I/O address space. It is divided by 8 by one 74HTC138 chip on my CPU board. Its CS signals are connected to the bus connector, so I can redirect them to the addition boards. The idea is to provide additional 74HTC138 chip on each board. This is the case with mentioned I/O board, which (among others) consists CF card. Decoder seems to work fine, because it also selects other I/O devices, such as UART or RTC which were at least partially tested. Only CF card causes problems.
I measured voltage at CF CS line when card should be inactive. It is 5V.
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There are other possible causes, of course. What's the clock rate of your computer?
Now it is 2MHz. I can be clocked down to 1MHz if necessary.
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Have you provided bypass capacitors?
Yes. There is 100nF capacitor near to VCC pin of every chip. Including CF card - in this case there is only one, because two VCC pins are close.
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And what's the construction technique -- are you using a solderless breadboard, for example?
It is mounted on two homemade, one sided PCBs. Here are the photos and schematics.