Speeding up a loop?


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  1. #1
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    Default Speeding up a loop?

    I'm trying to generate multiple audio tones using a DAC/resistor ladder (currently 4 bit / 16 values - will scale up eventually), but am troubleshooting increasing the speed of this loop for a single tone. Originally I was using a sine lookup table, but for multiple tones it appears to be too slow. I'm currently using a roughed-in binary version of 'Bhaskara I's sine approximation formula', which eliminates any lookups.

    However, the implementation I have currently is also slower than I'd like. I will scale up to a 20Mhz OSC at some point, but would like to get this loop <~25uS with a 4Mhz osc if possible.

    My question is: what instructions in the loop below are computationally expensive, and what changes could be suggested to speed it up? I'm not sure how much additional overhead I incur with word variable operations as opposed to byte variable operations...?

    Thanks in advance!
    Dave

    Code:
    define OSC 4
    
    cmcon=7
    
    timebyte    var byte
    x           var byte
    range       var word
    ampvar      var byte
    ampvar2     var byte
    nvar        var word
    dvar        var word
    fvar        var byte
    negflip     var byte
    
    pausec1     con 25
    
    TRISB = %00000000
    
    timebyte=0
    
    high porta.1
    
    loop1:
    x=timebyte & 127
    range=128-x
    range=range*x
    nvar=range<<3
    dvar=20480-range
    dvar=dvar>>2
    fvar=nvar/dvar
    ampvar=8+fvar
    negflip=2*timebyte.7
    ampvar2=ampvar-negflip*fvar
    PORTB=ampvar2
    timebyte=timebyte+1
    goto loop1

  2. #2

    Default Re: Speeding up a loop?

    There is no way that's anywhere near as fast as a lookup table.

    In your post on the MPELABS forum you had:
    Code:
     
    lookbyte=timebyte // 32
    ampvar=SINB[lookbyte]
    The killer there is the '//'.

    Did you try replacing that with richard's suggestion (anding timebyte with 31)?
    You need to get rid of all multiplication and division related instructions.

    If you want to get < 25us with a 4MHz clock you only have 25 asm instructions for the entire loop.

  3. #3
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    Default Re: Speeding up a loop?

    I did implement the "& 31" change, and that did significantly speed it up (output tone increased to ~2400 Hz, which is an order of magnitude increase and the // operation was indeed the main speed killer). I still have that on the back burner as a fallback if I can't get anything faster.

    At this point I'm trying to figure out if it's possible to implement multiple "decent" audio tones up to 10kHz on a 16Fxx architecture with a 20Mhz OSC. I've even considered compromising with a "less clean" sin approximation, such as a simple polynomial for the quarter wave (ex: =x*22/14-47/21249*x^2-7*x^3/(2^16) with x in binary radians has an error of about 1.7%, assuming I can implement it without being killed time-wise with DIV32).

    Thanks,
    Dave

  4. #4
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    Default Re: Speeding up a loop?

    (ex: =x*22/14-47/21249*x^2-7*x^3/(2^16) with x in binary radians has an error of about 1.7%, assuming I can implement it without being killed time-wise with DIV32)
    .

    on a esp8266 @ 160mhz clk or a esp32 , pic16fxx never
    This is more entertaining than Free to Air TV

  5. #5

    Default Re: Speeding up a loop?

    audio tones up to 10kHz on a 16Fxx architecture with a 20Mhz OSC
    16Fxx isn't the best choice

    For comparison:
    16Fxx @ 20MHz executes instructions at 5 MIPS,
    16F1xxxx @ 32MHz -> 8 MIPS,
    18F @ 64MHz -> 16 MIPS

    Looking back at your original pseudo code with lookup table SINB array in ram:
    Code:
    loop1:
    lookbyte=timebyte and 31
    ampvar=SINB(lookbyte)
    PORTB=ampvar
    timebyte=timebyte+1
    goto loop1
    On a PIC18F @ 64MHz this executes in 1us/loop (1MHz update rate)
    If you put the SINB table in ROM (which is more likely) it slows down to 1.25us/loop (800KHz update rate)

    I didn't do those evaluations using PBP, so I don't know if it's much slower than that.
    Last edited by tumbleweed; - 24th April 2020 at 11:18.

  6. #6
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    Default Re: Speeding up a loop?

    a [email protected] just passes 8khz with a 32 step lu in asm for a 4 bit r2r ladder on portb
    my lu values may be dodgy i just guessed them, result looks awful @ any freq


    Code:
    #CONFIG  __config  _INTOSC_OSC_NOCLKOUT & _WDT_ON & _PWRTE_OFF & _MCLRE_ON & _BODEN_ON & _LVP_OFF & _DATA_CP_OFF & _CP_OFF
    #ENDCONFIG
        
    DEFINE OSC 20 
    
    
    inx var byte  bank0
    trisb=%11110000
    goto overasm
    asm
        
    table
        addwf   PCL, F                                          
        retlw    8
        retlw 	 9
        retlw    10
        retlw 	 11
        retlw    12
        retlw 	 13
        retlw    14
        retlw 	 14
        retlw 	 15
        retlw    14
        retlw 	 14
        retlw 	 13
        retlw    12
        retlw 	 11
        retlw    10
        retlw 	 9  
        retlw    8
        retlw    6
        retlw 	 5
        retlw 	 4  
        retlw    3
        retlw    2
        retlw 	 1   
        retlw 	 1
        retlw 	 0
        retlw 	 1   
        retlw 	 1
        retlw    2
        retlw    3
        retlw 	 4
        retlw 	 5
        retlw    6     
    _lu  
        movf _inx ,w
        call table
        MOVE?AB PORTB
        RETURN
    ENDASM         
    overasm:
    inx=0
    loopp:
    CALL lu
     inx=inx+1
     inx =inx&31 
    goto loopp
    This is more entertaining than Free to Air TV

  7. #7
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    Default Re: Speeding up a loop?

    There is another approach. Check this video.

    https://www.youtube.com/watch?v=ophqt_RmiS0

    C
    ode in description.

  8. #8
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    Default Re: Speeding up a loop?

    This guy has done unbelievable things with low end PIC and also a color video game with multi channel sound and VGA output on a PIC18F2550!
    http://pic24.ru/doku.php/en/osa/articles/vga_game

    Using an RTOS...

    Ioannis

  9. #9
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    Default Re: Speeding up a loop?

    Yeah, in his creations, 16F690 outperforms some guys with RPI

  10. #10
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    Default Re: Speeding up a loop?

    I dare to say that he is a Case Study! Wish I was proficient in C to run through his code.

    Ioannis

  11. #11
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    Default Re: Speeding up a loop?

    in my wildest imaginings i never expected such feats on such modest hardware from a rtos.
    humbled yet again.
    rtos something else to learn where will it end?
    This is more entertaining than Free to Air TV

  12. #12
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    Default Re: Speeding up a loop?

    Who knows really? Very exciting though!

    Ioannis

  13. #13
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    Default Re: Speeding up a loop?

    Well there is a color game with sound even on 16F628A, I've built it, but you need SCART input to use it, no VGA output...

  14. #14
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    Default Re: Speeding up a loop?

    I think you mean this one:

    https://www.quinapalus.com/picsi.html

    Ioannis

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