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.

Attention!!! Note that the SCKx signal clock is not free running for normal SPI modes (8-bit or 16-bit). It will only run for 8 or 16 pulses when the SPIxBUF is loaded with data. It will however, be continuous for framed modes.

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 mikroBasic 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.

program SPITest

const  CS_PIN  = 0  ' DAC CS pin

dim value as word

sub procedure InitMain()
  TRISF.CS_PIN   = 0                  ' CS pin
  LATF.CS_PIN    = 1                  ' Set CS to inactive
  Spi1_Init_Advanced(_SPI_MASTER, _SPI_16_BIT, _SPI_PRESCALE_SEC_1, _SPI_PRESCALE_PRI_1,
  _SPI_SS_DISABLE, _SPI_DATA_SAMPLE_MIDDLE, _SPI_CLK_IDLE_HIGH,
  _SPI_ACTIVE_2_IDLE)                 ' Init SPI
end sub

' DAC increments (0..4095) --> output voltage (0..Vref)
sub procedure DAC_Output(dim valueDAC as word)

  while (SPI1STAT.1 = 1)              ' wait for SPI module to finish, if doing something
    nop
  wend
  LATF.CS_PIN = 0                     ' CS enable for DAC
  SPI1BUF = 0x3000 or valueDAC        ' Write CurrentValue to DAC (0x3 is required by DAC)
  while (SPI1STAT.1 = 1)              ' Wait for SPI module to finish write
    nop
  wend

    LATF.CS_PIN    = 1                ' CS disable for DAC
end sub

main:
  InitMain()
  while true
    value = 1
    while value < \$FFF
     DAC_Output(value)
     Delay_ms(5)
     value = value+1
    wend
  wend
end.

  Procedure 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 procedure Spi2_Init_Advanced instead of Spi2_Init should be used. The procedure 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 procedure Spi2_Write transmits data via the SPI module. Data reception is performed by the procedure Spi2_Read. The SPI moduled is disabled by the procedure Spi2_Stop (clearing the control bit SPIEN), and the SPI module enable (setting the control bit SPIEN) is performed automatically by the procedure Spi2_Init or Spi2_Init_Advanced.