From: Marko Mäkelä (marko.makela_at_hut.fi)
Date: 2003-01-27 19:03:21
On Sat, Jan 25, 2003 at 10:25:08PM -0500, William Levak wrote:
> Sounds like you have a bad data driver.
The Am29F040 chip in the prototype is partly directly connected to the
VIC-20 cartridge port. D0-D7 and A0-A12 are connected directly to the
cart port, while other signals go through the Lattice 2032 programmable
logic chip.
I just got the memory dump of the chip (made with a universal programming
device). After deshuffling the address and data bits in the image, I got
a file whose first 8k bank looks identical to what I read from the VIC-20.
All of the remaining 504 kilobytes are $ff. This suggests a software
problem.
The strange thing is that all other bytes except those $ff are written
correctly. And my program verifies each byte written by reading the same
address until the data matches. It appears that the bytes are changed later
by the program. I guess I'll have to apply binary search to narrow down
the address where the error occurs.
> I've never run into one where all the bits had to be high in order for the
> write to fail, but I have had drivers that failed when particular bits were
> high.
There is a pattern in the data: bits 0 through 2 (as seen by the VIC-20)
appear to be affected. On the 29F040, those would be bits 6, 7, and 5.
Marko
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