APU Frame Counter

The NES APU frame counter generates low-frequency clocks for the channels and an optional 60 Hz interrupt. The name "frame counter" might be slightly misleading because the clocks have nothing to do with the video signal.

The frame counter contains the following: divider, looping clock sequencer, frame interrupt flag.

Address	Bitfield	Description
$4017	MI--.----	Set mode and interrupt (write)
Bit 7	M--- ----	Sequencer mode: 0 selects 4-step sequence, 1 selects 5-step sequence
Bit 6	-I-- ----	Interrupt inhibit flag. If set, the frame interrupt flag is cleared, otherwise it is unaffected.
Side effects		The sequencer is restarted at step 1 of the selected mode. If mode is 1 the sequencer is then clocked, causing the first step to be carried out immediately. Finally, the divider is reloaded, resulting in a 1/240 second delay before the sequencer is next clocked.

The sequencer is clocked by the divider, which generates a 240 Hz (approx.) clock by dividing the master clock. The version of the CPU in NTSC and RGB systems divides the 21.47727 MHz system clock by 89490.

TODO: describe PAL

The frame interrupt flag is connected to the CPU's IRQ line.

Mode 0: 4-Step Sequence (bit 7 of $4017 clear)

Step	APU cycles	Envelopes & triangle's linear counter (Quarter frame)	Length counters & sweep units (Half frame)	Frame interrupt flag
1	7459	Clock
2	14915	Clock	Clock
3	22373	Clock
4	29830	Clock	Clock	Set if interrupt inhibit is clear
	29831			Set if interrupt inhibit is clear
	29832			Set if interrupt inhibit is clear
	240 Hz (approx.)	120 Hz (approx.)	60 Hz (approx.)

In this mode, the interrupt flag is set every 29830 CPU cycles, which is slightly slower than the 29780.5 CPU cycles per NTSC PPU frame.

Some Nintendo arcade boards, even those not directly based on the NES, use the 2A03 CPU as a sound processor. One of these is Punch-Out!!. This IRQ allows the CPU to keep time even if no PPU is connected to the bus.

Mode 1: 5-Step Sequence (bit 7 of $4017 set)

Step	APU cycles	Envelopes & triangle's linear counter (Quarter frame)	Length counters & sweep units (Half frame)
1	7459	Clock	Clock
2	14915	Clock
3	22373	Clock	Clock
4	29829
5	37283	Clock
	192 Hz (approx.), uneven timing	96 Hz (approx.), uneven timing

In this mode, the frame interrupt flag is never set.