*** M E M O R U M ***

Well, what else do you do with bits out of an Atari 2600...

Mark

Interesting choice of which 28 pins to pick for that CPU from the 40 on the 6502.

I can see why they left off the interrupts, programming the video chip must be hard enough with 100% control of the CPU timing.

I'm sure there was a better option than including the PHI2 out, that's another potential address line gone begging!.

If you go looking, there are numerous other versions of cut-down 6502 CPUs. The other one that was used was the 6504, same as the 6507 but with /IRQ replacing RDY. Apparently it was used in Commodore printers. But there were also versions with one less address pin. Giving a memory map of only 4K bytes! Needless to say, once memory prices dropped and therefore CPU prices also came down, these became obsolete. Assuming they were ever produced in the first place.

Purely due to the Atari 7800, the 6507 is the only common cut-down 6502 CPU.

Martin A wrote:I'm sure there was a better option than including the PHI2 out, that's another potential address line gone begging!.

Most likely the PHI2 was retained so that the minimum number of extra chips/gates were needed for a simple clock oscillator.
But yes, I agree, I would have produced a version with two extra address lines in preference to the RDY and PHI2 pins. Or maybe a version with one extra address line and /IRQ.

Oh, and while I'm remembering, the circuit in the photo in the top post is running on a 2MHz CPU clock. Despite the 6507 being officially only available as a 1MHz part... (at least according to the two different datasheets that I looked at).

Mark