Cothrell@DOCKMASTER.ARPA (09/16/87)
thanks for all the responses to my questions concerning the cpu in the later versions of the atari. to summarize the responses: 1. the CPU is indeed custom. 2. the custom part is internal to the 6502 and is responsible for tri-stating the address bus. 3. the "new" 6502 has pin 35 defined as "tri-state the bus" 4. everything else about the cpu appears to be standard 6502. now for my observations... I have built 2 processor cards(they drop into the cpu socket of my 1200xl), one with the circuitry of the old 800(from schematics) and one with a design of my own. Neither works...and I don't know why. I suspect the tri-state signal from the ANTIC. I have successfully duplicated/generated the bi-phase clock as well as the other circuitry. One of the potential problems is the wishy-washy signal levels I am getting(and generating) from the rest of the circuitry. the normal clock levels seem to be symetric about 1.25 V. The rest of the system seems to be little better. (the clocks have a swing of about .5V p-p) Perhaps the Atari chips are CMOS??? I dont know. The tri-state signal(HALT) from the ANTIC is by far the weirdest signal I have seen in digital electronics(ever!). Perhaps my scope is picking up harmonics or something, but I swear that the signal has at least 4 parts, one of which registers on my scope as a +5 flat-line. there are some high speed digital signals that register CLEARLY under the +5v with enough swing to definitely be on/off cycles. If anybody else can take a scope to their atari, check out pin 9 of the ANTIC or 35 of the CPU. I know that I am at the upper limit of my porta-scope when looking at that signal, but I can see enough to make me wonder just what all the activity is about. (just to be sure, I tried it on ac and dc couple, same picture). Anyway, they circuitry for the 800 seems to produce the cleanest signals, so I am saving that circuit...suggestions as to fixes, etc... are welcome. Scott Cothrell Cothrell at Dockmaster.Arpa