Tone quality
The quality of the DTMF tones is a bit troublesome. I used multimon (https://sourceforge.net/projects/multimon) sampling from my laptop's build in microphone to check the tones. When a tone is played once, it is often detected twice or thrice. This doesn't happen if I play tones from a Radio Shack #43-139 pocket dialer. The problems happen more often with 7, 8, 9, *, 0, and #.
I tried an Android DTMF decoder app called "DTMF Decoder". They all stink to varying degrees. This had problems decoding digits from the third column. Again, no problems with the Radio Shack pocket dialer.
I need to get this thing on an oscilloscope to get a better idea of what's going on.
Possible culprits:
- The low-pass filter after the output pin is inadequate, perhaps wrong corner frequency.
- 7-bit resolution isn't good enough. 8-bit should then be good enough. Then I'd have to output on two pins and mix on the board.