The mikroPacal 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.

  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
var SoftSpi_SDI : sbit at RC4_bit;            
var SoftSpi_SDO : sbit at RC5_bit;           
var SoftSpi_CLK : sbit at RC3_bit;           

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

Soft_SPI_Read

var data_read : word;
    data_send : byte;
...
// 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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program Soft_SPI;

// DAC module connections
var Chip_Select : sbit at RC0_bit;
    SoftSpi_CLK : sbit at RC3_bit;
    SoftSpi_SDI : sbit at RC4_bit;   // Note: Input signal
    SoftSpi_SDO : sbit at RC5_bit;

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

var value : word;

procedure InitMain();
  begin
    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
  end;

// DAC increments (0..4095) --> output voltage (0..Vref)
procedure DAC_Output( valueDAC : word);
var temp : byte; volatile;
  begin
    Chip_Select := 0;                        // Select DAC chip

    // Send High Byte
    temp := word(valueDAC shr 8) and 0x0F;   // Store valueDAC[11..8] to temp[3..0]
    temp := temp or 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
  end;

  begin
  
    ANSEL := 0;
    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 (TRUE) do                         // Endless loop
      begin

        if ((RA0_bit) and (value < 4095)) then   // If PA0 button is pressed
          Inc(value)                             //   increment value
        else
          begin
            if ((RA1_bit) and (value > 0)) then  // If PA1 button is pressed
              Dec(value);                        //   decrement value
          end;

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

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