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

[David Kelly]

On Tue, Nov 08, 2005 at 03:45:54PM +0000, Mike S. wrote:
> Hello to all,
> Can anyone tell me the best (faster) way to implement bit shifting
> (serial synch protocol -in a bit bang fashion-) with two general
> purpose digital pins (one pin for data the other for clock)? Using C
> is not fast enough! I need assembly!

Sounds like a homework assignment. Smart instructors monitor this list.

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
        }
}

Compiled with
"avr-gcc -O -gdwarf-2 -Wall -ffreestanding -mmcu=atmega64 -c junk.c"

Output of "avr-objdump -S junk.o":

myjunk(uint8_t byte) {
        uint8_t i;

        for( i = 0 ; i < 8 ; i++ ) {
   0:   90 e0           ldi     r25, 0x00       ; 0
                PORTA |= CLOCK_B;       //  rising edge of clock
   2:   d8 9a           sbi     0x1b, 0 ; 27
                if( byte & (1<<7) )
   4:   88 23           and     r24, r24
   6:   14 f4           brge    .+4             ; 0xc <myjunk+0xc>
                        PORTA |= BIT_B;         // set
   8:   d9 9a           sbi     0x1b, 1 ; 27
   a:   01 c0           rjmp    .+2             ; 0xe <myjunk+0xe>
                else
                        PORTA &= ~BIT_B;        // clear
   c:   d9 98           cbi     0x1b, 1 ; 27
                byte <<= 1;
   e:   88 0f           add     r24, r24
                PORTA &= ~CLOCK_B;      //  falling edge of clock
  10:   d8 98           cbi     0x1b, 0 ; 27
  12:   9f 5f           subi    r25, 0xFF       ; 255
  14:   98 30           cpi     r25, 0x08       ; 8
  16:   a8 f3           brcs    .-22            ; 0x2 <myjunk+0x2>
  18:   08 95           ret


Its not going to get much better than that.

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.