In the master mode, the clock provided to the SPI module is the instruction cycle TCY . This clock will then be prescaled by the primary prescaler, specified by PPRE<1:0> (SPIxCON<1:0>), and the secondary prescaler, specified by SPRE<2:0> (SPIxCON<4:2>). The prescaled instruction clock becomes the serial clock and is provided to external devices via the SCKx pin.

The SCKx clock frequency as a function of the primary and secondary prescaler settings is calculated by the following equation Examples of the SCKx frequencies as functions of the primary and secondary prescaler settings are shown in Table 9-1.

FCY = 30 MHZ		SECONDARY PRESCALER SETTINGS
		1:1	2:1	4:1	6:1	8:1
Primary prescaler settings
	1:1	30 000	15 000	7 500	5 000	3 750
	4:1	7 500	3 750	1 875	1 250	938
	16:1	1 875	938	469	313	234
	64:1	469	234	117	78	59
Fcy = 5 Mhz
Primary prescaler settings
	1:1	5 000	2 500	1 250	833	625
	4:1	1 250	625	313	208	156
	16:1	313	156	78	52	39
	64:1	78	39	20	13	10

Table 9-1 SCKx frequencies

NOTE: SCKx clock frequencies shown in kHz. All frequencies are not supported; electrical characteristics of individual microcontrollers of dsPIC30F family should be consulted.

Example:

The example shows the use of the specialized SPI library of the mikroC compiler for dsPIC microcontrollers which allows an easy initialization of the SPI module and write or read data from the transmit/receive buffer of the SPI module. The example shows the method of connecting the SPI2 module to the serial digital-to-analogue converter (DAC) MCP4921.

const char CS_PIN = 0;
unsigned int value;

void InitMain() {
 ADPCFG = 0xFFFF; // Set AN pins as digital

 Spi_Init(); // Initialize SPI module

 TRISF.CS_PIN = 0; // Set CS pin as output
}//~

// DAC increments (0..4095) --> output voltage (0..Vref)
void DAC_Output(unsigned int valueDAC) {
 char temp;

 PORTF.CS_PIN = 0; // Select DAC module

 // Send 2 bytes of valueDAC variable
 temp = (valueDAC >> 8) & 0x0F; // Prepare hi-byte for transfer
 // It's a 12-bit number, so only
 // lower nibble of high byte is used
 temp |= 0x30; // Set MCP4921 control bits
 Spi_Write(temp); // Send data via SPI

 temp = valueDAC; // Prepare lo-byte for transfer
 Spi_Write(temp); // Send data via SPI

 PORTF.CS_PIN = 1; // Deselect DAC module
}//~

void main() {
 InitMain();

 value = 2047; // When program starts, DAC gives
 // the output in the mid-range

 while (1) { // Main loop
 DAC_Output(value++);
 if (value > 4095)
 value = 0;
 Delay_ms(10);
 }
}//~!

Function Spi2_Init initializes the SPI module in master mode 8-bit formatted, without the SS2 pin, sampling in the middle of the SPI cycle, prescale 1:8, and the clock FCY:1 low while waiting. If another format or communication speed is desired, the function Spi2_Init_Advanced instead of Spi2_Init should be used. The function Spi2_Data_Ready reads the value of the status bit SPIRBF (SPI2STAT<0>) and checks if there are data loaded into the receive buffer register. The function Spi2_Write transmits data via the SPI module. Data reception is performed by the function Spi2_Read. The SPI moduled is disabled by the function Spi2_Stop (clearing the control bit SPIEN), and the SPI module enable (setting the control bit SPIEN) is performed automatically by the function Spi2_Init or Spi2_Init_Advanced.