Examination of SID triangle waveform
Data recorded on the triangle waveform ($10)
The data are recorded using a REU. The recording is done like this:
;* Program 1 *
;Turn off screen and sprites
...
;Set frequency of voice 3
LDA #<freq
LDX #>freq
STA $D40E
STX $D40F
;Reset waveform
LDA #$08 ;Set test bit
STA $D412
;Setup REU to sample register $D41B for $10000 cycles
...
;Start recording
LDA #$10 ;Waveform: Triangle
LDX #%1001000 ;REU-command: Start transfer
STA $D412 ;Start waveform
STX $DF01 ;Start transfer
...
Furthermore, a construct like this is used to get a value from the waveform
as fast as possible (via software) from the SID:
;* Program 2 *
...
LDA #$10
STA $D412 ;Start waveform
LDA $D41B ;Record early value
STA Haps ;Save value
...
The table below summarizes the recordings (all values in hex):
Frequency Haps REU
8000 04 05 06 07 ... FD FE FF FF FE ... 01 00 00 01 ...
4000 02 02 03 03 04 04 ...
2000 01 01 01 01 02 02 02 02 03 03 03 03 04 ...
1000 00 00 00 00 01 01 01 01 01 01 01 01 01 02 ...
FFFF 07 09 0B 0D 0F 11 13 15 17
DEAD 06 08 0A 0C 0D 0F 11 13 14 16 18 1A 1B 1D 1F 21 22 24 26
28 29 2B 2D 2E 30 32 34 35 37 39 3B 3C 3E 40 42 43 45
47 49 4A 4C 4E 50 51 53 55 56 58 5A 5C 5D 5F 61 63 64
Legend:
Frequency is the value stored to $D40E and $D40F.
Haps is the value of the Haps variable from program 2.
REU is the data collected with the REU after running program 1.
Conclusion: The triangle waveform starts with value $00 and goes
to $ff linearly. Then it goes from $ff to $00 linearly.
(As a sidenote, I'll mention that the data for the sawtooth
waveform at frequency $8000 is Haps= 02, REU= 02 03 03 04 04 ... FE FE FF
FF 00 00 01 01 ...)
The Haps variable is the value of the waveform on one cycle earlier than
the REU starts recording.
Further data used to investigate the frequency/waveform mapping:
Frequency
Haps+REU, Number of this*value
0001 7FFC*00 8000*01 8000*02 ...
0002 3FFC*00 4000*01 4000*02 ...
0003 2AA7*00 2AAB*01 2AAA*02 2AAB*03
0004 1FFC*00 2000*01 2000*02 ...
0005 1996*00 199A*01 1999*02 199A*03 1999*04
0006 1552*00 1555*01 1555*02 1556*03 1555*04 1555*05 1556*06
0007 1245*00 1249*01 1249*02 1249*03 1249*04 1249*05 1249*06 124A*07
1249*08
0008 0FFC*00 1000*01 1000*02 ...
0009 0E35*00 0E39*01 0E39*02
000A 0CC9*00 0CCD*01
000B 0B9F*00
Legend:
Frequency is the value stored to $D40E and $D40F.
Haps+REU is the contents of the Haps variable from program 2 and the contents
of the REU after running program 1.
The format is number of times of repeation * value. (i.e. 7FFC*00 means
that the value "00" repeated $7FFC times after each other in the
data. 0003*FE 0004*DE would mean this sequence of data was recorded: FE
FE FE DE DE DE DE. The first FE would come from the variable Haps, while
the rest would be obtained with the REU.)
Conclusion: We know that with the STA $D412; LDA $D41B construct,
A contains the value of the waveform after 4 cycles. This is consistence
with the observation that the number of times the value $00 appears generally
is 4 lower than the number of times the other values appear.
The overall behaviour can thus be simulated with this C-program:
void Triangle(long Frequency) {
long Cycle; /* Count of Phi2 cycles */
long Delay= 0x8000;
long Counter= 0; /* Initial value is 0 */
for (Cycle=0; ;Cycle= Cycle+1) {
/* Build waveform: Triangle */
if (Counter 256)
Waveform[cycle]= Counter;
else
Waveform[cycle]= 511- Counter;
/*
For the sawtooth waveform, use:
Waveform[Cycle]= Counter / 2;
*/
/* Handle frequency-function */
Delay = Delay- Frequency;
/* Frequency overflow? */
while (Delay =0 ) do {
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