After having someone post a 74 style MMC1 shematic for the NES, I trough I could come with something similar. After someone stated sound would nothing fit any simple programmable loigc chip in another thread, I came up with a sawtooth generator wich use no more than one 74HC160, three 74HC161, one op-amp and 11 resistors. It looks terrible because I've just rushed on it, but I think it's usable. You could control this channel with 8-bits for frequency, and one bit to enable/disable the channel. A frequency of $ff would mean 11.8 kHz, and a frequency of $00 would mean 43.7 Hz. The formula is just ( 1/(179*16*Value) = Frequency)
It's just a divider per 10 (because 1.7 MHz is too high, and I found 170k as a base of timing would be just right), then two daisy-chained 4-bits syncronous counters, wich are presettable by the programm. A high value mean they will fastly overflow, so high frequency. Then you have an actual counter wich create a frequency variable sawtooth and that outputs to a small hand-made 4bit DAC. Oh, I just forgot to add some logic to re-trigger the pre-scale counters. I'm just supid, but I want to improve this and come with something much much better until a few time if people is interested.
It's a very primary thing, but it could fit a few macrocells (?) in a programmable logic chip, and would require quite a few external componants (only the 74HCxxx could be inside as I understand things).
It's just a divider per 10 (because 1.7 MHz is too high, and I found 170k as a base of timing would be just right), then two daisy-chained 4-bits syncronous counters, wich are presettable by the programm. A high value mean they will fastly overflow, so high frequency. Then you have an actual counter wich create a frequency variable sawtooth and that outputs to a small hand-made 4bit DAC. Oh, I just forgot to add some logic to re-trigger the pre-scale counters. I'm just supid, but I want to improve this and come with something much much better until a few time if people is interested.
It's a very primary thing, but it could fit a few macrocells (?) in a programmable logic chip, and would require quite a few external componants (only the 74HCxxx could be inside as I understand things).
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Non-working shematic deleted from the internet by the author