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SPI timing issue

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theteaman

Joined: 04 Aug 2006
Posts: 98

SPI timing issue

Posted: Sun Oct 04, 2009 9:02 am

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Hi I have two PICs trying to communicate a few bytes over SPI.

Master:

Code:

while(1)
{
while(!input(flag));
while(input(flag));

spi_write(a);
spi_write(b);
}

Slave:

Code:

while(1)
{
output_high(flag);
delay_ms(1);
output_low(flag);

a = spi_read(0);
b = spi_read(0);
}

Now, this code all works fine. But when I add delays to after the WRITE in the master, like this:

Code:

while(1)
{
while(!input(flag));
while(input(flag));

spi_write(a);
delay_ms(1);
spi_write(b);
delay_ms(1);
}

the PIC seems to hang.. is there something I'm not seeing??

Any help appreciated. Thanks.

FvM

Joined: 27 Aug 2008
Posts: 2337
Location: Germany

Posted: Sun Oct 04, 2009 9:26 am

Reply with quote

Basically, performing spi_read() on the slave side doesn't synchronize receipt of multiple bytes. If you consult the CCS examples, they perform something like

Code:

while(!spi_data_is_in());
data = spi_read();

Utilisation of SPI receive interrupt would be the most versatile option in my opinion.

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