CPU Test Mode
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2A03E ???
The schematic for the Playchoice 10 permits the supervisor CPU to stop the 2A03E by driving pin 30 high, and then can reset it (both driving pin 30 low and 2A03E /RESET low). Testing pin 30 by itself (with a button) just crashes the CPU. It's unclear if it can be used for anything else.
2A03G / 2A03H 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 (same as value written to $4011)
W$401A: [L..T TTTT] - set state of triangle's sequencer to T, and lock all channels
(pulse+noise always output current volume, triangle/DPCM no longer advance) if L=1
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 11: 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; 0:counter stops while zero
|+--------- 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
