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

[Dave Hansen]

From: David Kelly <address@hidden>
[...]
> People keep saying "C isn't fast enough." I don't belive it. First
> attempt:
>
> #include <avr/io.h>
>
> #define CLOCK_B (1<<0)
> #define BIT_B   (1<<1)
>
> void
> 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
>                 else
>                         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;

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

        output_high(SPI_CLK);
        byte <<= 1;         // Shift acts as clock dwell delay
        output_low(SPI_CLK);

     } 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.

Regards,
  -=Dave