Brought to my attention by this post in the Hardware sub-forum, it seems that perhaps specific revisions of at least the 2A03 have extended APU functions by desoldering the 2a0x, bending out pin 30 -- or clipping it at the bottom and then bending it out -- and soldering a switch to it connected to both VCC and GND, and then supplying voltage to the pin. The only apparent downfall is the loss of controller inputs.
Correct me if I'm wrong but within Quietust's recent statements and hardware tests he's posted:
So basically, it adds 3 additional APU registers; and perhaps other functions as well?
I have a few questions of curiosity and points of interest to comment upon...
1.) The READ registers could potentially be used for demoscene productions and possibly music visualization. However, are the 7 bits on $401A reading decoded DPCM and raw 7-bit PCM data; or just decoded DPCM?
2.) Why would the triangle channel have a secret 4-bit PCM mode by writing to the first four bits of $401A? Also, how does changing the position of the triangle waveform affect a currently playing frequency? Is a crude volume control possible?
3.) What exactly is the function of bit 7 of $401A if it is intermittently set off and on during a song that is played? Does it turn all channels off and on or does it freeze APU function?
4.) Is there a potential wiring scheme that could enable these functions while maintaining controller input support?
5.) Which revisions of 2a03 support the extended functions? Does the PAL 2a07 contain the same "TEST" feature set?
Some food for thought and experimentation I suppose...
Correct me if I'm wrong but within Quietust's recent statements and hardware tests he's posted:
Quietust wrote:
Of very special note are the 4 signals at the very top for readable registers at $4018 (pulse 0 output on D0-D3 and pulse 1 output on D4-D7), $4019 (triangle output on D0-D3, noise output on D4-D7), $401A (DPCM output on D0-D6), and a writable register at $401A (set triangle position to D0-D4, and lock channel outputs using D7); all 4 of these signals have an additional enable which seems to come from the vicinity of pin 30.
So basically, it adds 3 additional APU registers; and perhaps other functions as well?
I have a few questions of curiosity and points of interest to comment upon...
1.) The READ registers could potentially be used for demoscene productions and possibly music visualization. However, are the 7 bits on $401A reading decoded DPCM and raw 7-bit PCM data; or just decoded DPCM?
2.) Why would the triangle channel have a secret 4-bit PCM mode by writing to the first four bits of $401A? Also, how does changing the position of the triangle waveform affect a currently playing frequency? Is a crude volume control possible?
3.) What exactly is the function of bit 7 of $401A if it is intermittently set off and on during a song that is played? Does it turn all channels off and on or does it freeze APU function?
4.) Is there a potential wiring scheme that could enable these functions while maintaining controller input support?
5.) Which revisions of 2a03 support the extended functions? Does the PAL 2a07 contain the same "TEST" feature set?
Some food for thought and experimentation I suppose...