AVR-Chat

I agree with David on the points he raised.  It would be very helpful if
Philippe could give us a little more detail as to how he has implemented his
bit banged SPI port.  Knowing what P4 and SPIOutBuf are used for and how
they are defined.

 

I once used this code to add an SPI port to an 8051
http://www.maximintegrated.com/app-notes/index.mvp/id/3524 .  Read the
application note carefully, especially the last sentence in the first
paragraph following the abstract.  It has been a while, but I believe that
it was quite fast.

 

Larry Viesse W7PAN

  

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of David Kelly
Sent: Tuesday, November 26, 2013 7:32 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Help with bit banged spi

 

  


On Nov 26, 2013, at 4:46 AM, Cagri Tanriover <dr_cagri_tanriover@yahoo.co.uk
<mailto:dr_cagri_tanriover@yahoo.co.uk> > wrote:

> P4 = ( *(SPIOutBuf + j) & 0x80 ? P4 | 1 : 0 );

I suggest looking at the compiler output listing in assembly language.

Philippe is using an 8051 variant so I doubt he is using gcc, but in my
observation with optimization enabled gcc does better with SPIOutBuf[j] than
with *(SPIOutBuf + j). Other compilers appreciate having the pointers broken
out manually, namely classic 68k compilers under Unix.

8051's have single-bit types and perhaps P4 has been defined as an output
bit. If so AND-ing and OR-ing is useless. Hopefully the compiler optimized
them out.

While we are at it this doesn't look right: P4 &= 0; That clears all bits in
P4. How about P4 &= ~1; which is 0b1111 1110 and only clears the zeroth bit
and is exactly as fast at run time? Of course that only matters if P4 is
larger than 1 bit wide. But if P4 is a bit variable then use P4 = 1; and P4
= 0;

I think it might be best to unroll the bit shifting. This also has the
possible advantage of not destroying the original buffer:

for( j=0 ; j<Num ; j++ ) {
Tmp = SPIOutBuf[j];

if( Tmp & (1<<7) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<6) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<5) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<4) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<3) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<2) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<1) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;

if( Tmp & (1<<0) )
P4 |= 1; 
else
P4 &= ~1;
SPICLK = 1;
PICLK = 0;
}

--
David Kelly N4HHE, dkelly@HiWAAY.net <mailto:dkelly@HiWAAY.net> 
============================================================
Whom computers would destroy, they must first drive mad.