62 FSK modem

62 : FSK modem

How it works

On the transmit side, it has two triangle wave synthesizers. The frequency of one can be altered by the data in pin, and the other one can be mixed in for DTMF.

On the receive side, it takes the input, heterodynes it with some square waves, and then filters I and Q over a 31 tap FIR filter. Finally, a phase detector is applied, that output is differentiated, and filtered for glitches. The noise immunity of this approach is questionable.

How to test

  • Set registers over SPI.
  • Connect the “incoming” phone line to a comparator, and to samples_in.
  • Connect the “outgoing” phone line to an R-2R DAC, and to samples_out.
  • Connect a serial port to data_in and data_out.
  • Enjoy squawking 300bps serial!


# Input Output
0 clock data_out
1 reset valid_out
2 cs_n samples_out[0]
3 sck samples_out[1]
4 mosi samples_out[2]
5 data_in samples_out[3]
6 none samples_out[4]
7 samples_in samples_out[5]