79. User port experimenting (3)
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Here is a little program to enable you to use your micro as an AF signal
 generator. The tone is emitted as a 5 Volt p-p squarewave on pin PB7 of the
User Port. I suggest you connect a 0.1uF capacitor and a 10k resistor in
series with the output, to protect the 6522 VIA chip. The program initially
generates 1000 Hz, and you then enter the frequency required. The timing
increments available are fairly coarse, especially near the 25kHz end, so the
frequency you actually get will not be exactly what you asked for. The
maximum frequency possible is 250kHz, but the next increment down is 167kHz,
which isn't very helpful! You can hear the tone by connecting a CRYSTAL
earphone or microphone insert across the output, or by feeding it into an
amplifier.

 10 ON ERROR GOTO 80
 20 ?&FE6B=&C0:?&FE64=&F2:?&FE65=1:freq%=1000
 30 REPEAT:F%=10^6/freq%/2-1.5:?&FE66=F% MOD 256:?&FE67=F% DIV 256
 40 actual%=10^6/((F%+2)*2)+0.5
 50 PRINT"Generating: ";actual%" Hz"'
 60 REPEAT:INPUT"Frequency ? "freq%:UNTIL freq%>49 AND freq%<25001
 70 UNTIL FALSE
 80 REPORT:PRINT" at line ";ERL:?&FE6B=0