This project increments a counter from 0 to 25,000,000 and back to zero again. The current value of this counter is then passed into a PDM modulator to generate the output. Filtering it with a low pass filter (designed for use in the audio range, recommended cutoff ~30 KHz) and then amplifying the result will result in usable audio.
Add a suitable RC low pass filter to output pin 7. This can be probed by an oscillocope as-is. To listen to the audio, the output of the RC filter should be attached to a suitable audio amplifier.
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | PCM sawtooth wave output (bit 9) | PCM sawtooth wave output (bit 9) | |
| 1 | PCM sawtooth wave output (bit 10) | PCM sawtooth wave output (bit 10) | |
| 2 | PCM sawtooth wave output (bit 11) | PCM sawtooth wave output (bit 11) | |
| 3 | PCM sawtooth wave output (bit 12) | PCM sawtooth wave output (bit 12) | |
| 4 | Frequency left shift amount (in bits) -- bit 0 | PCM sawtooth wave output (bit 13) | PCM sawtooth wave output (bit 13) |
| 5 | Frequency left shift amount (in bits) -- bit 1 | PCM sawtooth wave output (bit 14) | PCM sawtooth wave output (bit 14) |
| 6 | Frequency left shift amount (in bits) -- bit 2 | PCM sawtooth wave output (bit 15) | PCM sawtooth wave output (bit 15) |
| 7 | Frequency left shift amount (in bits) -- bit 3 | PDM sawtooth wave output (needs LPF) | PDM sawtooth wave output (needs LPF) |