topic Re: SPI communication between two MCF51QE128 in Processor Expert Software

SPI communication between two MCF51QE128

nlocatel — Wed, 08 Jun 2011 03:11:03 GMT

Hi everyone,

I'm facing a little trouble establishing a SPI communication between two MCF51QE128.

I used the CodeWarrior processor expert to make the appropriate beans for both, one is the master and the other is configured as a slave. The beans configurations are apparently correct, clock speed is valid, and electrical link between pins is verified.

But the slave is not executing the SPI Interruption Service Routine though clock and data are being sent from master to slave. The SPI interruption in the slave is, of course, enabled.

Can anyone help with this issue? I don't know what else to do.

You know what, it's solved! It was just the slave select timing, we had to pull it down much more in advance.

Thanks a lot.

Nicolas

Re: SPI communication between two MCF51QE128

armistej — Mon, 20 Jun 2011 20:48:57 GMT

Be sure to watch how fast you run your SPI interface.

There's a limit to how fast the slave device can accept data before it gets overrun. In a hybrid mode where the first SPI RX byte in a message is received via interrupts, and the rest are accepted by polling, I couldn't get any faster than 1MHz SPI clock when using a 24 MHz external clock on an MCF51CN128 part. Going to 2MHz and we got overruns occuring regularly. I didn't try to go any faster than 1MHz because our master only had a fixed choice of clock frequencies at whole MHz values.

Bottom line - for CPU to CPU SPI, the slave's ability to remove data from the SPI RX buffer is the limiting factor.

And yes, as you've discovered, slave select timing is important too. Choose your SPI mode carefully as they are not created equally.

Re: SPI communication between two MCF51QE128

nlocatel — Wed, 22 Jun 2011 02:40:44 GMT

Thanks for your comments, they helped.