SPI Loopback Example for F28377S Launchpad

Dear TI Team,

I am using the F28377S-LaunchPad XL with a simple SPI device.  I need to send 16 bits at a time to set a value in a potentiometer.  I started working with the example spi_loopack_cpu1 in ControlSuite to keep everything simple.

I checked the SCK output (GPIO 18 pin 76) with an oscilloscope and had 16 distinct peaks at ~3.2-3.3V.  I tried a few examples with sdata = 1, then = 3, then = 8 and everything looked fine.  But when I used a value of 43690 (binary: 1010101010101010),  the SPISIMOA (GPIO16 pin 36) had the following output:

As a result, I start losing values.  It seems to be able to handle 3 or 4 cycles up/down but any more and it drops to 1.8V.

Can you please advise what steps I can do to correct this?

Many thanks...

Source code that was used (no changes to library files) below:

#include "F28x_Project.h" // Device Headerfile and Examples Include File

// Prototype statements for functions found within this file.

// __interrupt void ISRTimer2(void);

void spi_xmit(Uint16 a);

void spi_fifo_init(void);

void spi_init(void);

int CSPin = 14;  // needed for external device

void main(void)

{

GPIO_SetupPinMux(CSPin, GPIO_MUX_CPU1, 0);

GPIO_SetupPinOptions(CSPin, GPIO_OUTPUT, GPIO_PUSHPULL);

GPIO_WritePin(CSPin, 1); // note circuit diagram 0V = light on; 3.3V light off

Uint16 sdata; // send data

Uint32 counter;

// Step 1. Initialize System Control:

// PLL, WatchDog, enable Peripheral Clocks

// This example function is found in the F2837xS_SysCtrl.c file.

InitSysCtrl();

// Step 2. Initialize GPIO:

// This example function is found in the F2837xS_Gpio.c file and

// illustrates how to set the GPIO to it's default state.

// InitGpio(); // Skipped for this example

// Setup only the GP I/O only for SPI-A functionality

// This function is found in F2837xS_Spi.c

InitSpiaGpio();

// Step 3. Clear all __interrupts and initialize PIE vector table:

// Disable CPU __interrupts

DINT;

// Initialize PIE control registers to their default state.

// The default state is all PIE __interrupts disabled and flags

// are cleared.

// This function is found in the F2837xS_PieCtrl.c file.

InitPieCtrl();

// Disable CPU __interrupts and clear all CPU __interrupt flags:

IER = 0x0000;

IFR = 0x0000;

// Initialize the PIE vector table with pointers to the shell Interrupt

// Service Routines (ISR).

// This will populate the entire table, even if the __interrupt

// is not used in this example. This is useful for debug purposes.

// The shell ISR routines are found in F2837xS_DefaultIsr.c.

// This function is found in F2837xS_PieVect.c.

InitPieVectTable();

// Step 4. Initialize the Device Peripherals:

spi_fifo_init(); // Initialize the Spi FIFO

spi_init(); // init SPI

// Step 5. User specific code:

// Interrupts are not used in this example.

sdata = 0;

counter = 0;

for(;;)

{

counter++;

if (counter > 500000)

{counter = 0;}

if (counter == 10000)

{GPIO_WritePin(CSPin, 1);} // CS line high - get ready to TX / load data register

if (counter == 400000)

{GPIO_WritePin(CSPin, 0);} // CS line low - get ready to receive data

// Transmit data

if (counter == 500000)  // simple delay

{

spi_xmit(sdata);  // stop in debug mode and change sdata to 43690

}

}  // for loop

} // main

// Step 7. Insert all local Interrupt Service Routines (ISRs) and functions here:

void spi_init()

{

SpiaRegs.SPICCR.all =0x000F; // Reset on, rising edge, 16-bit char bits

SpiaRegs.SPICTL.all =0x0006; // Enable master mode, normal phase,

// enable talk, and SPI int disabled.

SpiaRegs.SPIBRR.all =0x0063;

SpiaRegs.SPICCR.all =0x009F; // Relinquish SPI from Reset

SpiaRegs.SPIPRI.bit.FREE = 1; // Set so breakpoints don't disturb xmission

}

void spi_xmit(Uint16 a)

{

SpiaRegs.SPITXBUF=a;

}

void spi_fifo_init()

{

// Initialize SPI FIFO registers

SpiaRegs.SPIFFTX.all=0xE040;

SpiaRegs.SPIFFRX.all=0x2044;

SpiaRegs.SPIFFCT.all=0x0;

}

//===========================================================================

// No more.

//===========================================================================