40MHz just ISN'T fast enough!

[Guest]

I’ve moved to the 40MHz part. My code is extremely simple. I have a R2D ladder connected to port D, mapped to C variable AD . I need to bit bash a waveform out. The waveform is sampled at a rate of 48 samples per cycle. I need to output the waveform for precise amounts of time hence the 16bit “timer” loop variable. It all works superbly, just my PIC is a touch slow!

My code is very simple:

loop = 7600;
while(loop--) {
AD = triangle48[counter++];
if (counter==48) counter = 0;
} //while(loop--)

This generates:

.................... while(loop--) {
0096: MOVF 07,W
0098: MOVWF 03
009A: MOVF 06,W
009C: BTFSC FD8.2
009E: DECF 07,F
00A0: DECF 06,F
00A2: IORWF 03,W
00A4: BTFSC FD8.2
00A6: GOTO 00C6
.................... AD = triangle48[counter++];
00AA: MOVF 05,W
00AC: INCF 05,F
00AE: CLRF 03
00B0: CALL 0004
00B4: MOVWF 01
00B6: MOVFF 01,F8C
.................... if (counter==48) counter = 0;
00BA: MOVF 05,W
00BC: SUBLW 30
00BE: BTFSC FD8.2
00C0: CLRF 05
.................... } //while(loop--)
00C2: GOTO 0096

I am close; at 40MHz this gives me a frequency of 6.133kHz. I would really like 6.3kHz, so would like to trim a single instruction! Is this the most efficient code for the job? It looks quite compact to me, but hopefully some of you compact C hackers will spot a shortcut I don’t know!

[Ttelmah]

[Will Reeve]

It's stored in ROM, using a

unsigned int8 const triagle[48] {
...
};

statment which does not compile to "code" in the list file as far as I can see! It's one of those things I never thought about before a CCS black box I guess.

Keep well,

Will

[Will Reeve]

Me again,
I now have 8kHz

simple:
for (loop=0;loop<49;loop++)
ramlookup[loop] = triangle48[loop];
to move from ROM to RAM.

Which now gives me:
.................... while(loop--) {
00D2: MOVF 07,W
00D4: MOVWF 03
00D6: MOVF 06,W
00D8: BTFSC FD8.2
00DA: DECF 07,F
00DC: DECF 06,F
00DE: IORWF 03,W
00E0: BTFSC FD8.2
00E2: GOTO 0106
.................... AD = ramlookup[counter++];
00E6: MOVF 05,W
00E8: INCF 05,F
00EA: CLRF 03
00EC: ADDLW 08
00EE: MOVWF FE9
00F0: MOVLW 00
00F2: ADDWFC 03,W
00F4: MOVWF FEA
00F6: MOVF FEF,W
00F8: MOVWF F8C
.................... if (counter==48) counter = 0;
00FA: MOVF 05,W
00FC: SUBLW 30
00FE: BTFSC FD8.2
0100: CLRF 05
.................... } //while(loop--)
0102: GOTO 00D2

I can fine tune the frequency with the addition of a delay_cycles(x). Superb. Thanks alot. I wouldn't have thought of that! You have saved me a lot of head scratching!

[SteveS]

I'm not real hot at PIC assembly (which is why I use CCS!), but couldn't the "while(loop--)" code be replaced with assembly like this:

[Ttelmah]

[Aware]

If you use pointers instead of table lookup, it is even more faster.

Just store the variables in an array. And do the table lookup starting with the address of the variable and increment it when you need the next variable.


grt.

[Neutone]

[Guest]

I am using a PIC18F422 running directly from a 40MHz crystal which. I have a 40MHz signal on the OSC pins. The ICD debugger also reports the PIC to be running at 40MHz.

The following code:

.................... output_high(PIN_A0);
00CA: BCF F92.0
00CC: BSF F89.0
.................... output_low(PIN_A0);
00CE: BCF F92.0
00D0: BCF F89.0

Produces a square wave of frequency 1.666MHz on pin A0. According to my scope the “on” period is 200.0ns (5.000MHz)

I thought that a PIC running at 40MHz, has an instruction clock at 10MHz, so that’s right as it’s a two instruction job to turn on and off? Instruction 00CC sets it high; 00D0 sets it low, with one instruction in between? I can understand that.

Back to my waveform code; so each instruction takes 100.0ns. So the quickest I could ever hope to get the 48 (1 cycle of waveform) settings of PORTD would be 200ns x 48 = 9600ns = 104kHz?

Now I have 23 instructions x 48 = 1 cycle of my waveform so that takes 1104 instructions = 110400ns which should equal I should be getting 9.057kHz but I max out at 8kHz, 1kHz out?

Have I done that calculation correctly?

I am going to try pointers tomorrow and will let you all know how I get on, bedtime here in the UK!

Thanks everyone for your help. This forum is, as always, fantastic. My last job was a large Delphi program and it was a struggle without the support!

Keep well,

Will

[Ttelmah]

[Hans Wedemeyer]