Thanks. Technically this synth is quite simple, but that makes it exciting because it was more a hack (program loop synthesis) than optimized to precisely fulfil a predefined music theory.
You likely need to integrate over the pulse duration at pin 13 (hi=louder, lo=quieter) to mimic the capacitor for the volume envelope. The pulse width(?) from pin 13 controls the analogue decay rate (synth parameter C); the frequency is a fraction of the note pitch and additionally acts as a (often bass) suboscillator. The unused pin 7 disables the annoying APO so long it is pulled lo. It is important to add a DIP switch for this because APO makes the synth almost unusable.
Pin 17 goes hi only during the "train" effect noise and nowhere else (may be dead code from older software, or planned to switch a filter or light effect for a different toy). Pin 6 (wired to GND) disables the keymatrix out pins when pulled hi (i.e. no keys except the 2 power-on buttons work). Powering on with both pin 8 and 9 pulled lo starts in noise effect mode. Also pulling APO resistor from pin 12 hi (turns supply voltage on) during standby starts noise effect mode (hence can not be used to continue from standby with intact synth patch in RAM). Pin 11 stays always pulled low. Strange is that pin 1is wired hi trough a separate wire bridge (not PCB trace) despite pulling it lo has no effect; likely a prototype version used the eprom version PIC16C55 which uses this pin for eprom write mode.
Pin 10 rising edge (pull low and release) retriggers the analogue decay envelope. Holding pin 10 lo (through a resistor to avoid CPU damage) causes slow attack in all sounds.
Tones from pin 13, 15, 16, 17 are mixed through resistors to form the tone waveform. (pin13=R10 5.6k, pin 15=R13 68k, pin 16=R14 12k, pin 17=R15 1k). But pin 15 is only used during few sounds (button blips, power-on beeps, APO warning bass, effect noise "telephone"), so only 3 bits remain for the synth waveform, mixed from in synth mode 2 squarewaves and a complex multipulse. The result is routed through the analogue decay circuit.
