I2S Audio Host Mode of Operation

A typical application could be to play PCM data (8 kHz sample frequency, 16-bit data, 32-bit frame size) when interfaced to a codec client device. In this case, the SPI module is initialized to generate BCLK @ 625 kbps. Assuming a 20 MHz peripheral clock, FSPICLK = 20e6, the baud rate would be determined using Equation 19-2.

Equation 19-2. Baud Rate Calculation

Solving for the value of SPIxBRG is shown in Equation 19-3.

Equation 19-3. Baud Rate Calculation

The Baud Rate is now equal to 625e3. Equation 19-4 shows the resulting calculation.

Equation 19-4. Baud Rate Calculation

If the result of Equation 19-4 is rounded to the nearest integer, SPIxBRG is now equal to 15; therefore, the effective Baud Rate is that of Equation 19-5.

Equation 19-5. Baud Rate Calculation

The following steps can be used to set up the SPI module for operation in I2S Audio Host mode:

  1. If using interrupts, disable the SPIx interrupts in the respective IECx register.
  2. Stop and reset the SPI module by clearing the ON bit (SPIxCON1[15]).
  3. Reset the SPIx Control Register 1, SPIxCON1.
  4. Reset the SPIx Baud Rate Register, SPIxBRG.
  5. Clear the receive buffer.
  6. Clear the ENHBUF bit (SPIxCON1[0]) if using Standard Buffer mode or set the bit if using Enhanced Buffer mode.
  7. If using interrupts, perform these additional steps:
    1. Clear the SPIx interrupt flags/events in the respective IFSx register.
    2. Write the SPIx interrupt priority and sub-priority bits in the respective IPCx register.
    3. Set the SPIx interrupt enable bits in the respective IECx register.
  8. Clear the SPIROV bit (SPIxSTAT[6]).
  9. Write the desired settings to the SPIxCON1 register. The AUDMOD[1:0] bits (SPIxCON1[25:24]) must be set to ‘00’ for I2S mode and the AUDEN bit (SPIxCON1[31]) must be set to ‘1’ to enable the audio protocol.
  10. Set the SPIxBRG register to 0x0F (to generate approximately 625 kbps sample rate with SPICLK @ 20 MHz).
  11. Write the desired settings to the SPIxCON1 register:
    1. MSTEN (SPIxCON1[5]) = 1.
    2. CKP (SPIxCON1[6]) = 1.
    3. MODE[32,16] (SPIxCON1[11:10]) = 0 for 16-bit audio channel data.
    4. Enable SPI operation by setting the ON bit (SPIxCON1[15]).
  12. Transmission (and reception) will start immediately after the ON bit is set.

I2S Host Mode, 625 kbps BCLK, 16-Bit Channel Data, 32-Bit Frame

/* The following code example will initialize the SPI1 Module in I2S Host mode. */
_SPI1TXIP = 4;
SPI1STATbits.SPIROV = 0;       // clear the Overflow
SPI1BRG = 0x000F;              // to generate 625 kbps sample rate, SPICLK @ 20 MHz
SPI1CON1=0x80000460;           // AUDEN=1, I2S mode, stereo mode, 
                               // 16 bits/32 channel transfer, Host mode, CKP = 1
SPI1IMSKbits.SPITBFEN = 1;     // SPI1 transmit buffer full generates interrupt event
_SPI1TXIE = 1;                 // Enable interrupts
SPI1CON1bits.ENHBUF = 1;
SPI1CON1bits.ON = 1;
// from here, the device is ready to receive and transmit data