This decoder is fed by a RDS demodulator IC (or tuner) which has RDDA (data signal) and RDCL (clock signal) outputs.T he microcontroller is an Atmega168 clocked on the external 4.332MHz crystal from the RDS demodulator. The TDA7330B RDS demodulator IC from STMicroelectronics is connected to the Atmega168 with RDDA connected to PD4 (on PORTD) and RDCL to INT0 (PD2).
The tuner module on this card is an FM1216 from Philips. It has an unmodulated MPX FM signal output (AF sound output) which the RDS demodulator needs. This is the same signal which is fed into a stereo decoder to produce stereo sound outputs.
Atmega168 RDS decoder: [Link]