external memory problem

Jan 22, 2007 3 Replies
E

I am using a 89C450MCL mcu running at 29.492mhz With just the micro, 74F573 (or 74HCT573 tried both) and a IS62C1024-35 RAM chip it seems I loose the address sometimes at the output of the latch when /WR goes low. I am using plenty of bypas caps and 4.6uf caps for supply lines. I am simply trying to clear a memory range from 0000h - 00FFh. I put my logic probe on the address lines of the RAM chip after the latch and I can see the address sometimes going wrong. See

formatting link
for reference. As you can see I am just writing to memory and as the image shows 1Ch is fine 1Dh is fine but 1Eh and 1Fh are dropping the address then 20h is fine and 21h is fine. This is not the only place it goes wrong and I cannot find any corelation in the locations that go wrong. Any ideas??


Vote
Twitter Facebook LinkedIn
K

Not that I know the RAM, but it seems as if you might mix a TTL level device (both 74F573 and 74HCT573) with a CMOS device (IS62C1024-35) Look at the requirements for a HI input of the RAM and see if the TTL device will garantee that high voltage. Your logic probe does _not_ measure the exact voltage ! If that is causing the problem, you can "help" the TTL device by adding pull-up resistors in the output of the TTL (input CMOS device) That my idea Cheers

Ed skrev:

Vote
Twitter Facebook LinkedIn
M

Do you have pull-ups on P0?????? ( not another one surely :) )

Vote
Twitter Facebook LinkedIn
K

Rubbish! What I wrote was correct with the 74F device, but the 74HCT device should make the correct output. Then I would verify that the input on the xx573 also is latched to the output of the device. If not use pull-up on the input of the 573! or at least check the voltages during the latch pulse. More Cheers

Klaus (DK) skrev:

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required