JoeyG, Thanks a million for presenting working C code to do Goertzel!
The highest freq. in touchtone is 1633Hz while my freq. of interest to detect is 2kHz and I trying to understand how to make the c code modification without success.
A major consideration in the doorbell detector is battery usage. The 16F18456 doesn't have any hardware multiplier, which makes it 10x slower to do math than using any 18F MCU, and so the 16F18456 would use 10x the power and 10x the time?