Re: [avr-gcc-list] Speed challenge...

[Andreas Schwarz]

Hi Rob,

Friday, April 26, 2002, 4:32:01 AM, you wrote:

> Hi. I have a challenge. The (working) code below outputs 19 bits 
> serially from a 4433 uC to another chip using data, clock, and strobe 
> lines. The data comes from 3 lookup arrays of 256 bytes each. At 4MHz on 
> a 4433 uC , it takes about 87us to send all 19 bits. I wonder if there 
> is a faster way of doing the job?

you could do it in assembler. I have attached a function I wrote for a
SPI interface, perhaps this will help you.




/*

Funktion für Software-SPI
Sendet ein Datenbyte an einen an Port B angeschlossenen 74HC595

void spi(unsigned char);

*/

#include <macros.inc>
#include <ctoasm.inc>

#define value rP1

/*
Welche Pins sollen für die einzelnen SPI-Leitungen verwendet werden?
*/
#define MOSI 0
#define SCK 2


#define count rTI0

   .global spi
   .func spi

spi:
   clr count
   
loop:
   cbi PORTB, MOSI
   rol value
   brcc carrycleared
   sbi PORTB, MOSI
carrycleared:
        nop
   sbi PORTB, SCK
   nop
   cbi PORTB, SCK
   inc count
   cpi count, 8
   brne loop

   ret

   .endfunc

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