6 Dec
2008
6 Dec
'08
2:35 p.m.
Here is a technique to consider ..
I have some Dallas clock chips (1216E) that are designed to work under a
ROM chip. By definition it can't have any side effects on the ROM chip
that it is hiding underneath.
The Dallas trick is to get the programmer to 'wiggle' the address lines
with a known pattern before trying to talk to the clock chip. The
sequence is long enough such that it will probably never be encounted as
part of normal addressing for code reading the ROM. Once the magic
sequence is recognized, the clock chip intercepts reads to the ROM chip,
and also interprets further wiggling of the address lines as a write
operation.
If you use a technique like this you can get by without adding any extra
hardware.
Mike