topic Help with SPI - MC9S08GT32 in 8-bit Microcontrollers

Help with SPI - MC9S08GT32

juanb_arg — Sun, 28 Sep 2008 11:58:49 GMT

Hi everyone,
Well, i`m working with SPI in a proyect with MC9S08GT32 cpu
The problem i have is that i can`t find the facts that debugger gives me to be consistent with explanation of the datasheet.
First, after after writing the data register the one which sends data to the transmit register, the debugger always show me the value of the data register to be null.
Second, the secuence i see when I simule the next piece of code is,

sending secuence
while ( 1 ){

        while ( ! SPI1TF) ;

SPI1D =0xFF;                 /* writes the transmit buffer register of spi */

}

the transmit empty flag is set in 0 value after writing two consecutive bytes, and i think it should be set 0 value after 1 writing . Another problem is when the flag is finally set to 0, it never comes back to 1 value (ready to trasmit another byte)

I really think that the secuence to send a byte is, seeing if the transmit empty flag is set, then write the data register, and the is suposed that the thata is send after 8 spi clock times. and then repeat the cycle.

well the onlything i can see is that the SPI is not working, if someone can help me, send me a piece of code that has used succsesfuly, please i need some help

Added p/n to subject.

Message Edited by NLFSJ on 2008-09-29 12:09 PM

Re: Help with SPI

peg — Sun, 28 Sep 2008 12:09:47 GMT

Hello and welcome to the fora, Juan.

You can not read the SPI tx buffer. Attempting this gives you the receive buffer, RTFM.
See this thread for how to do it.

Re: Help with SPI

bigmac — Sun, 28 Sep 2008 13:20:57 GMT

Hello,

For C programs, the typical function for the master to transfer a single byte in each direction would be -

byte SPI_trans( byte val)

{

while (!SPI1S_SPTEF);

SPI1D = val; // Send byte value

while (!SPI1S_SPRF); // Wait for completion of transfer

return SPI1D;

}

The final read of SPI1D is necessary under all circumstances, to clear the SPRF flag and prevent an overrun condition. The manipulation of the SS output is external to the function. You probably will not be able to single step this code without disrupting its operation. This is because it is possible for the debugger itself to clear the hardware flags before the code can test the flag status.

To send each byte to the SPI slave device (and ignore the returned value), use the following call. The cast avoids a compiler warning because the returned value is not used.

(void)SPI_trans( byte_value);

To read the returned data from the slave, it is necessary to send a byte, usually a dummy value.

return_value = SPI_trans( 0); // Dummy send

Regards,

Mac

Message Edited by bigmac on 2008-09-28 04:30 PM

Re: Help with SPI

juanb_arg — Mon, 29 Sep 2008 05:25:44 GMT

Hi again!
Well i must say that i appreciate a lot your help, it is really useful.
I`ve done a little piece of code and tested it with full chip simulator, I think it`s going to work.
now i can see that i didnt understand prpeperly the manual...
well thanks again!

regards
Juan Bautista