MMC5 audio

Nintendo's MMC5 mapper provides extra sound output, consisting of two pulse wave channels and a PCM channel. The pulse wave channels behave almost identically to the native pulse channels in the NES APU.

The sound output of the square channels are equivalent in volume to the corresponding APU channels, but the polarity of all MMC5 channels is reversed compared to the APU.

Pulse 1 ($5000-$5003)

These registers manipulate the MMC5's first pulse wave channel, which functions the same as to those found in the NES APU except for the following differences:

• $5001 has no effect. The MMC5 pulse channels will not sweep, as they have no sweep unit.
• Frequency values less than 8 do not silence the MMC5 pulse channels; they can output ultrasonic frequencies.
• Length counter operates twice as fast as the APU length counter (might be clocked at the envelope rate).
• MMC5 does not have an equivalent frame sequencer (APU $4017); envelope and length counter are fixed to a 240hz update rate.

Other features such as the envelope and phase reset are the same as their APU counterparts.

Pulse 2 ($5004-$5007)

These registers manipulate the MMC5's second pulse channel.

PCM Mode/IRQ ($5010)

Write

7  bit  0
---- ----
Ixxx xxxM
|       |
|       +- Mode select (0 = write mode. 1 = read mode.)
+--------- PCM IRQ enable (1 = enabled.)

Read

7  bit  0
---- ----
Ixxx xxxx
|
+-------- IRQ (0 = No IRQ triggered. 1 = IRQ was triggered.) Reading $5010 acknowledges the IRQ and clears this flag.

Raw PCM ($5011)

This functions similarly to the NES APU's register $4011, except that all 8 bits are used.

Write

Writes are ignored in PCM read-mode.

7  bit  0
---- ----
WWWW WWWW
|||| ||||
++++-++++- 8-bit PCM data

Writing $00 to this register will have no effect on the output sound, and does not change the PCM counter.

PCM description

MMC5's DAC is changed either by writing a value to $5011 (in write mode) or reading a value from $8000-BFFF (in read mode). If you try to assign a value of $00, the DAC is not changed; an IRQ is generated instead. This could be used to read stream 8-bit PCM from ROM and terminate at $00.

IRQ operation

(pseudocode)

(On DAC write)
    if(value=0)
        irqTrip=1
    else
        irqTrip=0

(On $5010 write)
    irqEnable=value.bit7

(On $5010 read)
    value.bit7=(irqTrip AND irqEnable)
    irqTrip=0

Cart IRQ line=(irqTrip AND irqEnable)

Status ($5015, read/write)

This register is analogous to the APU Status register found within the NES at $4015, except that only the bottom 2 bits are used; being for the MMC5's two pulse channels.