SPI problem - PIC16f877a to MCP2515

[Chris b]

Hi,
i'm having a problem when using an SPI interface to communicate between a PIC16f877a and an MCP2515 CAN controller, using the CCS pic C compiler.

The following code is supposed to test the SPI interface by writing to a register in the MCP 2515, then reading it back. the problem is that some of the bits read back are different from those sent. I've put in a function to clear the BF bit (as suggested in the PIC16f877a datasheet) before the read takes place, but it doesnt make any difference.



while (TRUE)
{

for (k=0; k<=0xFF; k++)
{
output_low(CAN_CS); //begin SPI comms
spi_write(WRITE); //send the write instruction
spi_write(0x34); //address of a register to test reading and writing
spi_write(k); //send the data
output_high(CAN_CS); //SPI comunication finished
restart_wdt(); // restart the watchdog timer
delay_ms(50); //wait for 50ms before the read occurs

output_low(CAN_CS); //start another round of SPI comms
spi_write(READ); //send the read instruction
spi_write(0x34); //read from the register the data was written to


clear_BF(); //clear bit BF before reading the input data
i = spi_read(0x00); //read the value of the register into i
output_high(CAN_CS); //SPI comunication finished


i = i ^ k ; //XOR together to make bits where an error has happened = '1'

output_d(i); //output the errors onto LEDs for visual inspection
restart_wdt(); // restart the watchdog timer
delay_ms(50); //wait for 50ms before going round the loop again

}

}



the SPI port on the pic is configured as follows:

setup_spi(SPI_MASTER | SPI_L_TO_H | SPI_CLK_DIV_64 | SPI_XMIT_L_TO_H | SPI_SAMPLE_AT_END);

i've tried with various clock pre-scalers, sampling in the middle instead of the end, and opposite clock polarity, all to no avail.

i know the CAN controller is getting at least some SPI instructions, as i am able to change it's clock out pre-scaler via SPI successfully.

any suggestions?

[PCM programmer]

Try the test program shown in this thread.
http://www.ccsinfo.com/forum/viewtopic.php?t=18539

Note that he initially has a problem with the SPI interface
because he's using SPI Mode 2, and the MCP2515 data sheet
says that it must use mode 0 or 3. Later in the thread, he
changes his setup_spi() statement to use SPI mode 3.

It's easier to setup the correct SPI mode if you use these definitions.

[Chris B]

will give it a try, thanks

[chris b]

think i've got it sorted now, the other threads with explanation of modes etc were very useful.. a l
but what seems to have solved my problem was to specifically set TRIS<C> such that SDI pin of the PIC is configured as an input - something which the data sheet claims you don't have to do.
Thanks again for the help - a veritable resource of info this forum

:-)

[aswinthomas]

hi
i am also suffering with the same problem
i also want to set mode0,0 or mode1,1 as
per the data sheet ,i am doing this in assembly
can u explain me that which bit must be set
for it,i will be thankfull for it


aswin

[ckielstra]

[ASWIN]

HI CKIELSTRA,
THANKS for ur code ,i tried it in my code
but its not working i dont no wats the
mistake i here attched my code can u
tell me were the mistake or if possible
post me a code for spi_initialisation and
one for r/w for mcp2515 once again thanks



CS=0;
asm("nop");
SSPBUF=0B00000010;//write inst
SSPBUF=0X30;
SSPBUF=0b11100111;//data
CS=1;

delay(25000);


CS=0;
SSPBUF=0b00000011;//read inst
asm("nop");
SSPBUF=0X30;
asm("nop");
asm("nop");
HOLDER=SSPBUF;
CS=1;

asm("nop");
asm("nop");
asm("nop");
if(HOLDER==0b11100111)
{
PORTA=0X3F;
while(1)
{
}
}
while(1)
{
RA0=1;
}
}
//**************************************************************
void spi_interface (void)
{
TRISC =0b00010000; // setting SDO,SDI,SCK pins.SDI must set as input
//****************
//SSPSTAT
//****************
STAT_SMP=1; //Sample bit.Input data sampled at middle of data out put time
STAT_CKE=1; //SPI clock edge select. data trans on rising edge of SCK.case CKP=1.
//****************
//SSPCON
//****************
CKP=1; //Clock polarity select bit.Idle state of clock is a high level
SSPEN=1; //Synchronous serial port enable bit.
SSPM3=0; //SSPM3:SSPM0- Synchronous serial port mode select bit
SSPM2=0;
SSPM1=0; //SPI master mode, Fosc/4.
SSPM0=0;
}

aswin thomas

[ckielstra]

[aswin]

hi,
sorry man its of some posting
mistake i post my older code
once again sorry and thank u
for ur reply by

aswin