Software SPI Library

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.

Library configuration:

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

The following variables must be defined in all projects using Software SPI Library: Description : Example :
extern sfr sbit SoftSpi_SDI; Data In line. sbit SoftSpi_SDI at RC4_bit;
extern sfr sbit SoftSpi_SDO; Data Out line. sbit SoftSpi_SDO at RC5_bit;
extern sfr sbit SoftSpi_CLK; Clock line. sbit SoftSpi_CLK at RC3_bit;
extern sfr sbit SoftSpi_SDI_Direction; Direction of the Data In pin. sbit SoftSpi_SDI_Direction at TRISC4_bit;
extern sfr sbit SoftSpi_SDO_Direction; Direction of the Data Out pin sbit SoftSpi_SDO_Direction at TRISC5_bit;
extern sfr sbit SoftSpi_CLK_Direction; Direction of the Clock pin. sbit SoftSpi_CLK_Direction at TRISC3_bit;

Library Routines

Soft_SPI_Init

Prototype

void Soft_SPI_Init();

Returns

Nothing.

Description

Configures and initializes the software SPI module.

Requires

Global variables:

  • SoftSpi_SDI: Data in line
  • SoftSpi_SDO: Data out line
  • SoftSpi_CLK: Data clock line
  • SoftSpi_SDI_Direction: Direction of the Data in pin
  • SoftSpi_SDO_Direction: Direction of the Data out pin
  • SoftSpi_CLK_Direction: Direction of the Data clock pin
must be defined before using this function.
Example
// 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

Prototype

unsigned short Soft_SPI_Read(char sdata);

Returns

Byte received via the SPI bus.

Description

This routine performs 3 operations simultaneously. It provides clock for the Software SPI bus, reads a byte and sends a byte.

Parameters :

  • sdata: data to be sent.

Requires

Soft SPI must be initialized before using this function. See Soft_SPI_Init routine.

Example
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

Prototype

void Soft_SPI_Write(char sdata);

Returns

Nothing.

Description

This routine sends one byte via the Software SPI bus.

Parameters :

  • sdata: data to be sent.

Requires

Soft SPI must be initialized before using this function. See Soft_SPI_Init routine.

Example
// 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.

Copy Code To ClipboardCopy Code To Clipboard
// 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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