APU Frame Counter: Difference between revisions

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

Revision as of 20:01, 26 May 2012

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.

The sequencer is clocked on every other CPU cycle, so 2 CPU cycles = 1 APU cycle. The sequencer keeps track of how many APU cycles have elapsed in total, and each step of the sequence will occur once that total has reached the indicated amount. Once the last step has executed, the count resets to 0 on the next APU cycle.

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 After 2 or 3 CPU clock cycles*, the timer is reset.
If the mode flag is set, then both "quarter frame" and "half frame" signals are also generated.

* If the write occurs during an APU cycle, the effects occur 2 CPU cycles later, and if the write occurs between APU cycles, the effects occur 3 CPU cycles later.

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 3728 Clock    
2 7456 Clock Clock  
3 11185 Clock    
4 14914 Clock Clock Set if interrupt inhibit is clear
14914.5     Set if interrupt inhibit is clear
0 (14915)     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 3728 Clock  
2 7456 Clock Clock
3 11185 Clock  
4 14914    
5 18640 Clock Clock
0 (18641)    
  192 Hz (approx.), uneven timing 96 Hz (approx.), uneven timing

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