Sound working at very low cpu frequencies

assembly/bios/drivers/sn76489.asm

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+; TI SN76489 sound chip display driver
+; @author Daniele Verducci
+;
+; USAGE:
+; call Snd_init                     <-- inits sound (and silences default tone)
+; call Snd_beep                     <-- system beep
+
+; Sound card is on port 0
+SND_DATA_REG: EQU IO_0
+
+; Init device (silence all channels)
+; Bits meaning:
+; 1 R0 R1 R2 A0 A1 A2 A3
+;   Bit0 is 1
+;   Bit1,2,3 select the channel: 001, 011, 101, 111(noise)
+;   Bit4,5,6,7 selecy the attenuation (0000=full volume, 1111=silent)
+Snd_init:
+    ; silence ch1
+    ld a,%11111001
+    ;ld a,%10011111
+    out (SND_DATA_REG),a
+    ; silence ch2
+     ld a,%11111101
+    ;ld a,%10111111
+    out (SND_DATA_REG),a
+    ; silence ch3
+     ld a,%11111011
+    ;ld a,%11011111
+    out (SND_DATA_REG),a
+    ; silence noise ch
+    ld a,%11111111
+    out (SND_DATA_REG),a
+    ret
+
+Snd_beep:
+    ; ch1 max volume
+    ld a,%10010000
+    out (SND_DATA_REG),a
+    ; play beep freq
+    ld a,%10000000
+    out (SND_DATA_REG),a
+    ld a,%00001000
+    out (SND_DATA_REG),a
+    ; wait 1 sec
+    ld bc, 10
+    call Time_delay55
+    ; silence ch1
+    ld a,%10011111
+    out (SND_DATA_REG),a
+    ret