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SPI communication

 
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PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Wed Jul 30, 2008 4:32 pm     Reply with quote
This chip has a really simple SPI interface. When you shift in the
control byte, you are shifting out the status byte at the same time.
So you can do it two different ways:

You can do it with two lines of spi code, like this:
Code:

int8 control_byte;
int8 diagnosis_data;

control_byte = 0x55;

spi_write(control_byte);
diagnosis_data = spi_read();


Or you can do it with one line, like this:
Code:

int8 control_byte;
int8 diagnosis_data;

control_byte = 0x55;

diagnosis_data = spi_read(control_byte);


These two methods produce the same ASM code in the .LST file.

Note that 0x55 is just a representative value. You need to choose
the real value.

-----------------
I just noticed something about this chip. It claims to be an SPI interface
but it sends the LSB (bit 0) first. But the hardware SPI module in the
PIC sends and receives the MSB (bit 7) first.

There are two possible solutions to this. You can switch to using
software SPI and send the data LSB first. Or, you can use hardware
SPI as shown in the code above, but then you need to call a bit reversal
routine to convert both the transmitted data and the received data.

There are several bit reversal routines shown in the Code library:
http://www.ccsinfo.com/forum/viewtopic.php?t=23364

The revised code would look like this:
Code:
spi_write(reverse_bits(control_byte));
data = reverse_bits(spi_read());

I have just used the function name of 'reverse_bits' as a generic name
for any of the routines shown in the Code Library.
rdesgagn



Joined: 30 Jul 2008
Posts: 3

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PostPosted: Wed Jul 30, 2008 4:39 pm     Reply with quote
Thanks a lot PCM Programmer. Me too, I have noticed that the H-Bridge is sending the LSB first and I was reading it backwards so the data didn't make sense at all.

Thank you again for your help, everything is working like I want !!!
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