Thanks for the replies, guys. If you remember my posts from any of the other forums, you'll know that I'm primarily an old analog engineer. I have a fair amount of logic design experience, but none with uC's.
I'm actually planning to use delta modulation (search CVSD) for the A/D, which yields a one bit audio stream (think AM radio for audio quality). I started out looking at a 256kX1 Cypress SRAM, but it dissipates too much power for my liking. I'm thinking of using a 32kX8 lower power part instead, and since I only have one data bit, I think I can use 3 bits of the 18 bit address bus to demux the data input and mux the output, effectively making it a 256kX1 part. Does that make sense?
Brent, I was planning to read and then write to the same address, then increment the address. The delay would be controlled by the "length" (circumference?) of the circular register. That way I wouldn't need separate read and write addresses, with a variable offset.
I'll take a look at the ATMega64, and the 8051's also. Thanks for the suggestions.
Regarding fast accesses - 10usec sounds slow to me, even for a microcontroller. Am I mistaken?
I like the rotary encoder idea, but I'll bet it ain't as cheap as a center-off, momentary SPDT switch.
Plus, I think the human interface would be better. I would need a pretty stiff detent on an encoder (maybe they come that way?). With the switch, I would need to select one of 3 voltages with the switch, and feed the result to two comparators or to an A/D input.
Thanks again, guys. If you or others have more input for me, I'm open to it.