8085 Microprocessor inadvertent addressing

Thread Starter

petewh

Joined Mar 23, 2016
45
I'm working with the 8085 for hobby stuff. I'm using a standard 74HCT138 decoder and the straight thru 74HCT573 version of the 373. Having built a few I notice at the end of instruction cycles I get an unexpected higher order address. Not from my software. And the lengthier the program the more of them.
The micro is not reading or writing at those times. So I guess it's OK but I'd like to know what it is. The problem has not caused difficulty with whatever I am having the micro do so far as I know.
I'm using wiring not printed circuit and tried different schemes. I don't think it's stray pickup or noise from improper decoupling. Was the 8085 known for a DON'T CARE address state? I thought I would reach out to the Forum and see if someone remembers the 8085. Honestly, I'd like to put the issue to bed.
 

JohnInTX

Joined Jun 26, 2012
4,281
The upper address lines in the diagrams in section 2 show that A15 - A8 are driven but 'UNSPECIFIED' so you might indeed see something there that looks out of place. As long as you have your upper address latch correct and decoding on the latched values you should be good.
On some older Intel uP like the 8048 and I think the 8008, they output the accumulator on those spare cycles for debugging purposes. It may be for some diagnostic purpose on the 8085 but I don't know for sure.

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Thread Starter

petewh

Joined Mar 23, 2016
45
The opcode fetch diagram you sent certainly has the info specified that I was looking for as an explanation. Thanks for the book download- I don't have that one. Yes I believe I am OK except I have to scope some more as best I can. It seems that I have some unaccounted for decoding out of the 138. Since there isn't a read or a write going on at those times no harm done but the idea that my chip select is active seems wrong to me. It may be just something to live with.
 

JohnInTX

Joined Jun 26, 2012
4,281
Thanks for the book download- I don't have that one
You might find these useful as well. The SDK manual describes an early proto/demo board and has a software listing of a resident monitor that uses a terminal to control/inspect/debug stuff. Along with some custom hardware, I used various versions of monitors for years as a primary development tool.
Have fun. i8085 is a forgotten jewel.
 

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