PIC Math - the need for speed

[Darrel Taylor]

I determined it was 48 cycles by using this procedure ...

instruction execution time
http://www.picbasic.co.uk/forum/showthread.php?t=365

I've double checked, and it's definately 48. Although my hardware is different, 18F452 @ 20Mhz, it shouldn't make that much difference.

For your test routine I get these results:

Value = 20000
Start:                     ' Cycles   uS@20mhz
    PORTD.0 = 1            '    1         .2
    Delay = Value /*$004D  '   48        9.6
    PORTD.0 = 0            '    1         .2
    PauseUs 700            ' 3501      700.2   
Goto Start                 '    2         .4
                             -----     -----
              ' Loop Total   3553      710.6

And, this I didn't expect. Here's the results from Ingvar's first example

                            ' Cycles   uS@20mhz
Dummy1 = OldPulseTime >> 2  '  31        6.2
Delay = Dummy1 >> 2         '  31        6.2
Delay = Delay + Dummy1      '   4         .8
                             -----     -----
              '      Total     66       13.2

But, his second one looks pretty quick. As long as .3125 is close enough.

                            ' Cycles   uS@20mhz
Dummy1 = OldPulseTime >> 1  '   5        1.0
Dummy1 = Dummy1 >> 1        '   3         .6
Delay = Dummy1 >> 1         '   5        1.0
Delay = Delay >> 1          '   3         .6
Delay = Delay + Dummy1      '   4         .8
                             -----     -----
              '      Total     20        4.0

[HenrikOlsson]

Darrel, Ingvar,
Hmm, this is interesting.
I tried Ingvars 1'st code and measured it to 82uS. Then I tried the second one and measured it to 22uS which is great! 31.25% works fine for me.

I'm still wondering though why the numbers doesn't match. Ingvars first code should take 66uS but I measure it to ~80uS. His second one should take 20 and it does, close enough. Then we have Darrel's 48 cycle code that takes ~250uS on my hardware. I just verified that again to make sure.

I would think my scope was out of calibration if it wasn't for the fact that 700uS pause I have measures correct on the scope.

The application hardware is 12F629 but the testing above is on a 16F877 @ 4MHz.

Thanks guys!
/Henrik Olsson.

[Darrel Taylor]

Apparently, your Scope works Really Well.

I ran the numbers again for a 16F877 this time, and the results are rather surprising.

Start:                     ' Cycles   uS@20mhz
    PORTD.0 = 1            '    1         .2
    Delay = Value /*$004D  '  243       48.6
    PORTD.0 = 0            '    1         .2
    PauseUs 700            ' 3502      700.4
Goto Start                 '    4         .8
                             -----     -----
           '    Loop Total   3751      750.2
           ' Without Pause    249

                            ' Cycles   uS@20mhz
Dummy1 = OldPulseTime >> 2  '  37        7.4
Delay = Dummy1 >> 2         '  37        7.4
Delay = Delay + Dummy1      '   6        1.2
                             -----     -----
              '      Total     80       16.0

                            ' Cycles   uS@20mhz
Dummy1 = OldPulseTime >> 1  '   5        1.0
Dummy1 = Dummy1 >> 1        '   3         .6
Delay = Dummy1 >> 1         '   5        1.0 
Delay = Delay >> 1          '   3         .6
Delay = Delay + Dummy1      '   6        1.2
                             -----     -----
              '      Total     22        4.2

I really didn't expect that much difference between the same code compiled for the 2 different chips.

Way to go Ingvar.

[HenrikOlsson]

Darrel,
Wow, thanks!
I agree, that's a bit strange isn't it? It's gonna be interesting to see how fast it will run on the 12F629 where it actually will reside and operate when it's finished. That timing routine of yours seems to come in handy from time to time!

Thanks again!
/Henrik Olsson.

[Ingvar]

Darrel,
You forget that your 18F uses it's hardware multiplier. The 16 series does not have one and needs to do a software multply, that's a HUGE difference. I'm a little surprised that the ">>2" takes that much longer than ">>1". I suspected around 10-20 cycles difference, not 60. It should take a little longer since it uses a looped code rather than the straight code ">>1" produces. Strange, perhaps i'll look into it sometime ............ or not.

Henrik,
You should make sure that the variables reside in the same memorybank. You do that by using the "BANK" statement. If you don't it will be slower since the pic needs to switch banks between each statement. Declare your variables like .....

Dummy1          VAR WORD BANK0
Delay           VAR WORD BANK0
OldPulseTime    VAR WORD BANK0

/Ingvar

PS. Henrik, vart skall jag skicka fakturan DS.

[Darrel Taylor]

Right Ingvar,

That's what it is, the Hardware Multiplier.

The funny thing is that a couple of years ago, I did a comparison between 16F and 18F multiplication, and found that PBP didn't use the hardware MUL for 18F's. So, I made my own MUL routine for 18F's and have been using it ever since (when I need the speed). I hadn't looked since then, but, meLabs must have snuck it in there when I wasn't paying attention. Nothing listed in the Version History.

That's good to know!
<br>

P.S. Ingvar, Add some to that bill for me too.

[Ingvar]

Sure, as soon as Henrik tells me where to send it