The mikroC PRO for PIC provides routines for implementing Software SPI communication. These routines are hardware independent and can be used with any MCU. The Software SPI Library provides easy communication with other devices via SPI: A/D converters, D/A converters, MAX7219, LTC1290, etc.

• SPI to Master mode
• Clock value = 20 kHz.
• Data sampled at the middle of interval.
• Clock idle state low.
• Data sampled at the middle of interval.
• Data transmitted at low to high edge.

The library configures SPI to the master mode, clock = 20kHz, data sampled at the middle of interval, clock idle state low and data transmitted at low to high edge.

  Important : The Software SPI library implements time-based activities, so interrupts need to be disabled when using it.

External dependencies of Software SPI Library

Library Routines

• Soft_SPI_Init
• Soft_SPI_Read
• Soft_SPI_Write

Soft_SPI_Init

// Software SPI module connections
sbit SoftSpi_SDI at RC4_bit;
sbit SoftSpi_SDO at RC5_bit;
sbit SoftSpi_CLK at RC3_bit;

sbit SoftSpi_SDI_Direction at TRISC4_bit;
sbit SoftSpi_SDO_Direction at TRISC5_bit;
sbit SoftSpi_CLK_Direction at TRISC3_bit;
// End Software SPI module connections
...
Soft_SPI_Init(); // Init Soft_SPI

Soft_SPI_Read

unsigned short data_read;
char data_send;
...
// Read a byte and assign it to data_read variable
// (data_send byte will be sent via SPI during the Read operation)
data_read = Soft_SPI_Read(data_send);

Soft_SPI_Write

// Write a byte to the Soft SPI bus
Soft_SPI_Write(0xAA);

Library Example

This code demonstrates using library routines for Soft_SPI communication. Also, this example demonstrates working with Microchip's MCP4921 12-bit D/A converter.

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// DAC module connections
sbit Chip_Select at RC0_bit;
sbit SoftSpi_CLK at RC3_bit;
sbit SoftSpi_SDI at RC4_bit;
sbit SoftSpi_SDO at RC5_bit;

sbit Chip_Select_Direction at TRISC0_bit;
sbit SoftSpi_CLK_Direction at TRISC3_bit;
sbit SoftSpi_SDI_Direction at TRISC4_bit;
sbit SoftSpi_SDO_Direction at TRISC5_bit;
// End DAC module connections

unsigned int value;

void InitMain() {
  TRISA0_bit = 1;                        // Set RA0 pin as input
  TRISA1_bit = 1;                        // Set RA1 pin as input
  Chip_Select = 1;                       // Deselect DAC
  Chip_Select_Direction = 0;             // Set CS# pin as Output
  Soft_SPI_Init();                       // Initialize Soft_SPI
}

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

  Chip_Select = 0;                       // Select DAC chip

  // Send High Byte
  temp = (valueDAC >> 8) & 0x0F;         // Store valueDAC[11..8] to temp[3..0]
  temp |= 0x30;                          // Define DAC setting, see MCP4921 datasheet
  Soft_SPI_Write(temp);                  // Send high byte via Soft SPI

  // Send Low Byte
  temp = valueDAC;                       // Store valueDAC[7..0] to temp[7..0]
  Soft_SPI_Write(temp);                  // Send low byte via Soft SPI

  Chip_Select = 1;                       // Deselect DAC chip
}

void main() {

  ANSEL  = 0;                            // Configure AN pins as digital
  ANSELH = 0;
  C1ON_bit = 0;                          // Disable comparators
  C2ON_bit = 0;
  
  InitMain();                            // Perform main initialization

  value = 2048;                          // When program starts, DAC gives
                                         //   the output in the mid-range

  while (1) {                            // Endless loop

    if ((RA0_bit) && (value < 4095)) {   // If RA0 button is pressed
      value++;                           //   increment value
      }
    else {
      if ((RA1_bit) && (value > 0)) {    // If RA1 button is pressed
        value--;                         //   decrement value
        }
      }

    DAC_Output(value);                   // Send value to DAC chip
    Delay_ms(1);                         // Slow down key repeat pace
  }
}

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