TMS320F28377S: SPI not working as expected

Part Number: TMS320F28377S

Hi all,

I have tried to implement a basic SPI single character read/write without complete success.  I can provide more details if necessary, but let me start off with the basics and we'll see if I can uncover my mistake.

Setup:  I'm using a USB to SPI converter (MCP2210) to send out a character on the SPI channel to the MCU slave.  In firmware, on Rx interrupt, I read the input and then write a character (of whatever value I assign in the debug watch window).  No FIFO buffers.  All muxes are correctly turned on.  Interrupt code is: 

__interrupt void spiRxIsr(void)
{
	SpicRegs.SPIFFRX.bit.RXFFOVFCLR=1;  // Clear Overflow flag
	SpicRegs.SPIFFRX.bit.RXFFINTCLR=1;  // Clear Interrupt flag
	PieCtrlRegs.PIEACK.all |= M_INT6;   // Issue PIE ACK

	spiRxData = SpicRegs.SPIDAT;   //SpiRxData is Uint16
	SpicRegs.SPIDAT = spiTxData;   //spiTxData is Uint16
}

So the results are odd for both the value read and the value written (and read on my PC).  It appears that the the SPIDAT register doesn't get fully flushed after each read and so the values I'm writing in continue to shift bit by bit.  It is inconsistent even, and for several read/write cycles the data read and sent is correct.  Let me know if I'm missing something or should try something in particular.  I have spent a while and tried many combinations of things without seeing an improvement.

Here's my initializing code:

//
// InitSPI - This function initializes the SPI to a known state
//

void SPI_InitSpiC(void)
{
// Initialize SPI-C

// Set reset low before configuration changes
// Clock polarity (0 == rising, 1 == falling)
// 8-bit character
// Disable loop-back
SpicRegs.SPICCR.bit.SPISWRESET = 0;
SpicRegs.SPICCR.bit.CLKPOLARITY = 0;
SpicRegs.SPICCR.bit.SPICHAR = (7); //8 bit transmision
SpicRegs.SPICCR.bit.SPILBK = 0;

// Enable master (0 == slave, 1 == master)
// Enable transmission (Talk)
// Clock phase (0 == normal, 1 == delayed)
// SPI interrupts are enabled
SpicRegs.SPICTL.bit.MASTER_SLAVE = 0;
SpicRegs.SPICTL.bit.TALK = 1;
SpicRegs.SPICTL.bit.CLK_PHASE = 1;
SpicRegs.SPICTL.bit.SPIINTENA = 1;

// Halting on a breakpoint will not halt the SPI

SpicRegs.SPIPRI.bit.FREE = 1;

// Release the SPI from reset
SpicRegs.SPICCR.bit.SPISWRESET = 1;
}

//
// InitSpicGpio - Initialize *** GPIOs
//
void SPI_InitSpiCGpio()
{
EALLOW;

//
// Enable internal pull-up for the selected pins
//
// Pull-ups can be enabled or disabled by the user.
// This will enable the pullups for the specified pins.
// Comment out other unwanted lines.
//
GpioCtrlRegs.GPCPUD.bit.GPIO69 = 0; // Enable pull-ups
GpioCtrlRegs.GPCPUD.bit.GPIO70 = 0;
GpioCtrlRegs.GPCPUD.bit.GPIO71 = 0;

//
// Set qualification for selected pins to asynch only
//
// This will select asynch (no qualification) for the selected pins.
// Comment out other unwanted lines.
//
GpioCtrlRegs.GPCQSEL1.bit.GPIO69 = 3; // Asynch input
GpioCtrlRegs.GPCQSEL1.bit.GPIO70 = 3;
GpioCtrlRegs.GPCQSEL1.bit.GPIO71 = 3;

//
//Configure SPI-A pins using GPIO regs
//
// This specifies which of the possible GPIO pins will be SPI functional
// pins.
// Comment out other unwanted lines.
//

//G-Mux
GpioCtrlRegs.GPCGMUX1.bit.GPIO69 = 3; // MOSI
GpioCtrlRegs.GPCGMUX1.bit.GPIO70 = 3; // MISO
GpioCtrlRegs.GPCGMUX1.bit.GPIO71 = 3; // SCLK

//Mux
GpioCtrlRegs.GPCMUX1.bit.GPIO69 = 3; // MOSI
GpioCtrlRegs.GPCMUX1.bit.GPIO70 = 3; // MISO
GpioCtrlRegs.GPCMUX1.bit.GPIO71 = 3; // SCLK

EDIS;
}