switching between EPROMS

[Kalle Pihlajasaari]

Hi,

>   If you switch when the uP is not reading or writing (look at uP docs), 
> AND the uP doesn't copy the tables into another RAM chip (for speed..?), 
> there shouldn't be any problems.

In addition to the simple switching I described you would have to 
be very carefull if there are word constants stored in the area that
will change between versions as you may switch between the two byte 
reads and read a number that is way out of bounds.

old number
   0123  = 291 dec

new number
   00e8  = 232 dec

one old byte and one new byte

   01e8  = 488 dec
   0023  =  35 dec

These may be out of bounds and cause a jump table to do the
wrong thing (code dependant) and only if the change is between the
two bytes in a word.  There may obviously be congruency problems
with the rest of the data unless the changes are kept small.

You could improve the hardware I described by gating one of the low
address lines with the latch signal so you will only switch when 
fetching in low memory (processor dependant).

This will add another IC at least to the one or two.  Switch debouncing
will not be a significant problem if the data is allowed to be a 
mix of more than one version during the time the switch bounces, every
read will be a good read but from a indeterminate version for a few
read cycles.

Cheers.
-- 
Kalle Pihlajasaari     kalle at data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751