Re: Motorolla MC 68HC705J5A programming problems

From: Tobias Scharnberg (
Date: 06/23/04

Date: Wed, 23 Jun 2004 14:29:21 +0200

What a great idea! The CPUs program memory is big enough to hold approx.
2 different programs and FF opcode is STX for this CPU, so thats no
problem. I programmed it with an offset and the CPU does run. But the
next programming in the region below does not work. It seems like the
Motorola Programmer does write the memory from the very beginning, no
matter if FFs are written or not. :-(

Dusko Lolic schrieb:

> You could have saved 19 OTPs and maybe all 20 of them.
> What I used to do back in the old days of OTPs & EPROMs was to put small
> test programs at the top of unused program memory. CPU would just run
> over unused FFs repeatedly executing harmless OP code until it runs into
> actual code. New program would go just in front of the old one and so on
> until the whole device is full, but I would stop before that and leave
> enough memory so that device could be actually used in a less code size
> demanding project. In the end you run your tests and not a single OTP
> device has to be wasted. :-)
> Of course, it depends on what FF op code means to the given CPU. I don't
> know what it does on 68HC705 but on Z80 it is RST 38h and I managed to
> fool it, just to avoid erasing EPROMs too often.
> Thank you God for giving us flash! :-)