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Re: [avr-gcc-list] AVR assembly for fast bit bang

From: Dave Hansen
Subject: Re: [avr-gcc-list] AVR assembly for fast bit bang
Date: Wed, 09 Nov 2005 09:00:58 -0500

From: David Kelly <address@hidden>
People keep saying "C isn't fast enough." I don't belive it. First

#include <avr/io.h>

#define CLOCK_B (1<<0)
#define BIT_B   (1<<1)

myjunk(uint8_t byte) {
        uint8_t i;

        for( i = 0 ; i < 8 ; i++ ) {
                PORTA |= CLOCK_B;       //  rising edge of clock
                if( byte & (1<<7) )
                        PORTA |= BIT_B;         // set
                        PORTA &= ~BIT_B;        // clear
                byte <<= 1;
                PORTA &= ~CLOCK_B;      //  falling edge of clock

[...snip assembly...]

It might be tough to do better on AVR. My standard SPI routine uses a do-while loop, which might save an instruction or two, but made about a 30% difference on the PIC compiler I used. Something like

  void output_spi_byte(uint8_t byte)
     uint8_t bit_ctr = 8;

        output_low(SPI_DATA);   // Set data bit low...
        if (byte & 0x80)
           output_high(SPI_DATA);   // ...or high, as required

        byte <<= 1;         // Shift acts as clock dwell delay

     } while (--bit_ctr);

I don't have avr-gcc handy to see if it's any better than your code. I was more concerned with size than speed, as you may be able to tell. If speed is the ultimate object, unrolling the loop will help.


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