DSPIC33 SPI Master chip select line not working

Question

Ive been trying really hard to get the SPI bus working on a DSPIC33EP128GM604. Ive configured SPI3 as a standard 8 bit master module and have designated a chip select pin so I can toggle slave devices.

I cannot get the chip select line to work properly, it seems as if the DSPIC does not wait for transmission to complete before toggling the chip select line from LOW to HIGH. Ive checked the Erratas and nothing seems relevant to this problem. Furthermore ive tried to swap the CS line to some other pin. Ive also tried SPI module 1, but problem persists.

Here is my code:

SPI 3 configuration:

void Init_SPI3 ( void )
{
IFS5bits.SPI3IF = 0; // Clear the Interrupt flag
IEC5bits.SPI3IE = 0; // Disable the interrupt
// SPI1CON1 Register Settings
SPI3CON1bits.DISSCK = 0; // Internal serial clock is enabled
SPI3CON1bits.DISSDO = 0; // SDOx pin is controlled by the module
SPI3CON1bits.MODE16 = 0; // Communication is word-wide (16 bits)
SPI3CON1bits.SSEN = 0; //No CS
SPI3CON1bits.MSTEN = 1; // Master mode disabled
SPI3CON1bits.PPRE = 2; //Pre-scaler
SPI3CON1bits.SPRE = 8;//Pre-scaler 2
SPI3CON1bits.SMP =1; // Input data is sampled at the middle of data output time
SPI3CON1bits.CKE =0; // Serial output data changes on transition from
SPI3CON1bits.CKP = 0; // Idle state for clock is a low level;
SPI3STATbits.SPIEN = 1; // Enable SPI module
// Interrupt Controller Settings
IFS5bits.SPI3IF = 0; // Clear the Interrupt flag
IEC5bits.SPI3IE = 1; // Enable the interrupt
}

SPI 3 Pin configuration:

TRISBbits.TRISB10 = 0; //Set SPI3CLK as output
TRISBbits.TRISB11 = 0; //Set SPI3DATAOUT as output

RPOR4bits.RP42R=32;  //SPI3_CLK out
RPOR4bits.RP43R=31;  //SPI3_DATA out

Chip select line pin configuration:

TRISBbits.TRISB4 = 0; //SPI3 CS is on Pin B4, set it to Output

Simple main routine:

    while(1==1){

    PORTBbits.RB4 = 0;//Set CS line to low  
   WriteSPI3(0xaa); //This is from the peripheral library to be safe....
  PORTBbits.RB4 = 1; //Set CS line to high
                    }

But here is the output:

Answer 1

From the datasheet and example codes, the Receive Buffer Flag is sufficient to tell us when the TX event is complete and thus the CS can toggle. It is a bit strange why there isnt a flag for this chip that tells us when the SPI shift register has transmitted all its data. All the example code only check the Receive buffer flag.

After a lot of head scratching, trial and error and finally the microchip forums, i can confirm that this code works:

   uint8_t Write_SPI1(uint8_t command) { //Does work    
    SPI1BUF = command; // send data over SPI
    while(!SPI1STATbits.SPIRBF) ; //wait until SPIRBF goes high  
    return SPI1BUF; //KEY STATEMENT THAT MADE A DIFFERENCE, Return the actual data received
    }

Along with this code, if your peripheral depends on Peripheral Pin Select (SPI2,SPI3 etc), it is important to define not only a SPI Clock Out but also a SPI Clock In even if you are in master mode

However, what is rather baffling to me is that this code (that i think should do the same thing) does not work:

  void Write_SPI1(uint8_t command) { //Does not work, premature toggle of CS, tried this code initially....
    uint8_t garbage;
    SPI1BUF = command; // send data over SPI
    while(!SPI1STATbits.SPIRBF) ; //wait until SPIRBF goes high 
    garbage = SPI1BUF; // read dummy data, does not seem to clear the flag as expected
    }

Answer 2

Wait until the SPI transmission is complete before un-asserting the chip select with something like

 while(SPI3_Tx_Buf_Full);  //wait till completion of transmission

See the example using SPI1 at http://pat.cybersites.ca/docs/PIC24F/SPI_Example1.html