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fceux/documentation/tech/exp/vrcvi.txt

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moved fceu to its own folder to make room for other projects [[Split portion of a mixed commit.]]
2006-07-29 05:46:15 +00:00
VRCVI CHIP INFO
----- ---- ----
By:
Kevin Horton
khorton@iquest.net
The RENES Project:
Reverse-engineering
the world.
V1.01 08/31/99 teeny fix
V1.00 08/31/99 Complete Version
VRCVI (VRC6) (48 pin standard 600mil wide DIP)
------------
This chip is used in such games as Konami's CV3j and Madara. It's unique
because it has some extra sound channels on it that get piped through the
Famicom (note this is a fami-only chip and you will not find one in any
NES game). "VI" of "VRCVI" is "6" for the roman numeral challenged.
This chip generates its audio via a 6 bit R2R ladder. This is contained
inside a 9 pin resistor network like so:
3K 3K 3K 3K 3K 2K
/------*-\/\/-*-\/\/-*-\/\/-*-\/\/-*-\/\/-*------*-\/\/-\
| | | | | | | | |
\ \ \ \ \ \ \ | |
/ / / / / / / | |
\ 6K \ 6K \ 6K \ 6K \ 6K \ 6K \ 6K | |
/ / / / / / / | |
| | | | | | | | |
O O O O O O O O O
GND D0 D1 D2 D3 D4 D5 Aud In Aud Out
Legend:
-------
(s) means this pin connects to the System
(r) this only connects to the ROM
(w) this is a SRAM/WRAM connection only
AUD : these pass to the resistor network
CHR : these connect to the CHR ROM and/or fami's CHR pins
PRG : these connect to the PRG ROM and/or fami's PRG pins
WRAM : this hooks to the WRAM
CIRAM : the RAM chip which is on the fami board
.----\/----.
GND - |01 48| - +5V
AUD D1 - |02 47| - AUD D0
AUD D3 - |03 46| - AUD D2
AUD D5 - |04 45| - AUD D4
(s) PRG A12 - |05 44| - PRG A16 (r)
(s) PRG A14 - |06 43| - PRG A13 (s)
(s) M2 - |07 42| - PRG A17 (r)
(r) PRG A14 - |08 41| - PRG A15 (r)
*1 (s) PRG A1 - |09 40| - PRG A13 (r)
*1 (s) PRG A0 - |10 39| - PRG D7 (s)
(s) PRG D0 - |11 38| - PRG D6 (s)
(s) PRG D1 - |12 37| - PRG D5 (s)
(s) PRG D2 - |13 36| - PRG D4 (s)
(r) PRG /CE - |14 35| - PRG D3 (s)
(s) R/W - |15 34| - PRG /CE (s)
*2 (w) WRAM /CE - |16 33| - /IRQ (s)
(r) CHR /CE - |17 32| - CIRAM /CE (s)
(s) CHR /RD - |18 31| - CHR A10 (s)
(s) CHR /A13 - |19 30| - CHR A11 (s)
(r) CHR A16 - |20 29| - CHR A12 (s)
(r) CHR A15 - |21 28| - CHR A17 (r)
(r) CHR A12 - |22 27| - CHR A14 (r)
(r) CHR A11 - |23 26| - CHR A13 (r)
GND - |24 25| - CHR A10 (r)
| |
`----------'
VRCVI
*1: On some VRCVI carts, these are reversed. This affects some registers.
*2: This passes through a small pulse shaping network consisting of a
resistor, diode, and cap.
Registers: (sound related only)
----------
regs 9000-9002 are for pulse channel #1
regs a000-a002 are for pulse channel #2
regs b000-b002 are for the phase accumulator channel (sawtooth)
(bits listed 7 through 0)
9000h: GDDDVVVV
D - Duty Cycle bits:
000 - 1/16th ( 6.25%)
001 - 2/16ths (12.50%)
010 - 3/16ths (18.75%)
011 - 4/16ths (25.00%)
100 - 5/16ths (31.25%)
101 - 6/16ths (37.50%)
110 - 7/16ths (43.75%)
111 - 8/16ths (50.00%)
V - Volume bits. 0000b is silence, 1111b is loudest. Volume is
linear. When in "normal" mode (see G bit), this acts as a general
volume control register. When in "digitized" mode, these act as a
4 bit sample input.
G - Gate bit. 0=normal operation, 1=digitized. In digi operation,
registers 9001h and 9002h are totally disabled. Only bits 0-3 of
9000h are used. Whatever binary word is present here is passed on
as a 4 bit digitized output.
9002h: FFFFFFFF
F - Lower 8 bits of frequency data
9003h: X---FFFF
X - Channel disable. 0=channel disabled, 1=channel enabled.
F - Upper 4 bits of frequency data
A000h-A002h are identical in operation to 9000h-9002h. One note: this chip
will mix both digitized outputs (if the G bits are both set) into one
added output. (see in-depth chip operation below)
B000h: --PPPPPP
P - Phase accumulator input bits
B001h: FFFFFFFF
F - Lower 8 bits of frequency data
B002h: X---FFFF
X - Channel disable. 0=channel disabled, 1=channel enabled.
F - Upper 4 bits of frequency data
How the sounds are formed:
--------------------------
This chip is pretty cool. It outputs a 6 bit binary word for the sound
which is passed through a DAC and finally to the NES/Fami. Because of this,
the sound can be emulated *very* close to the original.
I used my scope to figure all this out as well as my meter and logic probe
so it should be 100% accurate.
Block diagrams of the VRCVI: (as reverse engineered by me)
----------------------------
| F bits | | D bits| | V bits |
| (12) | | (3) | | (4) |
\___________/ \_______/ \________/
+-----+ +----------------+ +-----------+ +------------+
| | | | | | | |--\
| OSC |-->|Divider (12 bit)|-->| Duty Cycle|-->| AND array |(4)> chan out
|(M2) | | | | Generator | | |--/
+-----+ +----------------+ +-----------+ +------------+
^ ^
| |
| |
G X
One Pulse channel (both are identical)
--------------------------------------
How it works: The oscillator (in the NES, the clock arrives on the M2 line
and is about 1.78Mhz) generates our starting frequency. This is passed
first into a divide by 1 to 4096 divider to generate a base frequency.
This is then passed to the duty cycle generator. The duty cycle generator
generates the desired duty cycle for the output waveform. The "D" input
controls the duty cycle generator's duty cycle. If the "G" bit is
a "1", it forces the output of the duty cycle generator to a "1" also. If
the "X" bit is "0", it forces the output of the duty cycle generator to "0",
which effectively disables the channel. Note that this input has precidence
over the "G" bit.
The AND array is just that- an array of 4 AND gates. If the output of
the duty cycle generator is a "0", then the "chan out" outputs will all be
forced to "0". If the output of the duty cycle generator is a "1", then
the chan out outputs will follow the V bit inputs.
Note that the output of this generator is a 4 bit binary word.
---
| F bits | | P bits|
| (12) | | (6) |
\___________/ \_______/
+-----+ +----------------+ +-----------+
| | | | | |--\
| OSC |-->|Divider (12 bit)|-->| Phase |(5)> chan out
|(M2) | | | |Accumulator|--/
+-----+ +----------------+ +-----------+
^
|
|
X
The Sawtooth (ramp) channel
---------------------------
This one is pretty similar to the above when it comes to frequency selection.
The output frequency will be the same relative to the square wave channels.
OK, the tough part will be explaining the phase accumulator. :-) What it is
is just an adder tied to a latch. Every clock it adds a constant to the
latch. In the case of the VRCVI, what you do is this:
The ramp is generated over the course of 7 evenly spaced cycles, generated
from the divider. Every clock from the divider causes the phase accumulator
to add once. So... let's say we have 03h in the P bits. Every 7 cycles
the phase accumulator (which is 8 bits) is reset to 00h.
cycle: accumulator: chan out: notes:
-----------------------------------------
0 00h 00h On the first cycle, the acc. is reset to 0
1 03h 00h We add 3 to 0 to get 3
2 06h 00h We add 3 to 3 to get 6
3 09h 01h
4 0ch 01h
5 0fh 01h
6 12h 02h
7 00h 00h Reset the acc. back to 0 and do it again
This will look like so: (as viewed on an oscilloscope)
- - - 2
--- --- --- 1
--- --- --- 0
|
012345601234560123456-+
Note: if you enter a value that is too large (i.e. 30h) the accumulator
*WILL WRAP*. Yes, this doesn't sound very good at all and you no longer
have a sawtooth. ;-)
The upper 5 bits of said accumulator are run to the "chan out" outputs.
The lower 3 bits are not run anywhere.
"X" disables the phase accumulator and forces all outputs to "0".
Note that the output of this generator is a 5 bit word.
---
Now that the actual sound generation is out of the way, here's how the
channels are combined into the final 6 bit binary output:
+---------+
| Pulse |
|Generator|
| #1 | Final 6 Bit
+---------+ Output
| (4) | / \
\ / | (6) |
+---------+ +---------+ +---------+
| 4 Bit |--\ | 5 Bit | /--|Sawtooth |
| Binary |(5)>| Binary |<(5)|Generator|
| Adder |--/ | Adder | \--| |
+---------+ +---------+ +---------+
/ \
| (4) |
+---------+
| Pulse |
|Generator|
| #2 |
+---------+
Channel Combining
-----------------
The three channels are finally added together through a series of adders
to produce the final output word. The two pulse chans are most likely added
first since they are 4 bit words, and that 5 bit result is most likely
added to the sawtooth's output. (The actual adding order is not known,
but I can make a *very* good guess. The above illustrated way uses the least
amount of transistors). In the end it does not matter the order in which
the words are added; the final word will always be the same.
The final 6 bit output word is run through an "R2R" resistor ladder which
turns the digital bits into a 64 level analog representation. The ladder
is binarally weighted and works like the DAC on your soundcard. :-)
(so take heart emulator authours: just run the finished 6 bit word to
your soundcard and it will sound right ;-).
Frequency Generation:
---------------------
The chip generates all its output frequencies based on M2, which is
colourburst divided by two (1789772.7272Hz). This signal is passed
directly into the programmable dividers (the 12 bit frequency regs).
Squares:
--------
These take the output of the programmable divider and then run it through
the duty cycle generator, which in the process of generating the duty cycle,
divides the frequency by 16.
To calculate output frequency:
1789772.7272
Fout = ----------------
(freq_in+1) * 16
This will tell you the exact frequency in Hz. (note that the * 16 is to
compensate for the divide by 16 duty cycle generator.)
Saw:
----
This is similar to the above, however the duty cycle generator is replaced
with a phase accumulator which divides the output frequency by 14.
To calculate output frequency:
1789772.7272
Fout = ----------------
(freq_in+1) * 14
This will tell you the exact frequency in Hz. (note that the * 14 is to
compensate for the phase accumulator.)
So how accurate is this info, anyways?
--------------------------------------
I believe the info to be 100% accurate. I have extensively tested the
output of the actual VRCVI chip to this spec and everything fits perfectly.
I did this by using a register dump and a QBASIC program I wrote which
takes the register dump and produces a WAV file. All frequency and
duty cycle measurements were taken with a Fluke 83 multimeter, and all
waveform data was culled from my oscilloscope measuring the real chip.
---EOF---