CPU Test Mode

2A03E / 2A03G Test Mode

Pin 30 of the 2A03 revision G can be activated to enable a special test mode for the APU. This activates registers for testing the APU at $4018-$401A at the expense of deactivating the joypad input registers at $4016-$4017:

R$4018: [BBBB AAAA] - current instant DAC value of B=pulse2 and A=pulse1 (either 0 or current volume)
R$4019: [NNNN TTTT] - current instant DAC value of N=noise (either 0 or current volume) and T=triangle (anywhere from 0 to 15)
R$401A: [.DDD DDDD] - current instant DAC value of DPCM channel
W$401A: [L..T TTTT] - "lock channel output" and set current instant phase of triangle
                      (What is the polarity of L? How does it work?)

The schematic for the Playchoice 10 implies that this functionality is also in the 2A03E. There it is used by the supervisor CPU to lock out controller input until coins are given to the arcade machine.

2A03letterless Programmable Interval Timer

Visual inspection of the original 2A03 revision indicates there was planned IRQ functionality from $401C-$401F, but this was left unfinished and unusable and was outright removed from later CPU revisions.

W$401C,D,E: write lower, middle, upper bits of counter reload value
R$401C,D,E: read lower, middle, upper bits of current counter value
W$401F: [ELAC DTZC]
         |||| ||||
         |||+----+-- Clock source
         |||  |||    00: M2÷16   10: M2÷256
         |||  |||    01: Rising edges of JOY2  10: Falling edges of JOY2
         |||  ||+--- Exists but does nothing. Toggles (invisibly) on terminal count.
         |||  |+---- Value driven out on JOY1. Toggles on terminal count.
         |||  +----- 1:count up; 0:count down
         ||+-------- 1:automatic reload when counter reaches 0
         |+--------- 1:reload immediately
         +---------- 1:IRQ enabled (counts regardless)
R$401F: [E... ...I]
         |       |
         |       +-- Timer IRQ would be asserted if enabled
         +---------- IRQ is enabled
        Reads from $401F acknowledge the IRQ

See also

• Forum: Breaking NES apart (WARNING: traffic)
• Forum: Memory map and 2A03 register map / 2A03 cutting-room floormetal
• See: File:Apu address.jpg
• See: CPU pin out and signal description