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SPI in PIC (SPI Module of PIC18F4550 – a practical guide)

The Serial Peripheral Interface (SPI) is one of the fastest and most commonly used communication protocols in embedded systems. With just four lines, SPI in PIC (PIC18F4550) enables high-speed, full-duplex communication between the PIC18F4550 microcontroller and devices like sensors, displays, memory chips, and DACs.

In this blog, we’ll walk through configuring the SPI module of the PIC18F4550, understanding master-slave roles, and creating a simple project example

🎯 Goal: After reading this post, you’ll be able to set up I/O pins, blink LEDs, and read switch states — the essential first step to mastering embedded systems.

What is SPI Communication?

SPI (Serial Peripheral Interface) is a synchronous serial communication protocol designed for short-distance, high-speed data transfer between a master and one or more slave devices.

It uses the following lines:

SS (Slave Select) – Enables specific slave device

SCK (Serial Clock) – Clock signal generated by the master

SDI (Serial Data In) – Data line for receiving

SDO (Serial Data Out) – Data line for transmitting

SPI Pins in PIC18F4550

The SPI module in PIC18F4550 is implemented through the MSSP (Master Synchronous Serial Port) peripheral.

Step 1: Configuring SPI as Master

We’ll configure the MSSP module to operate as the SPI Master, enabling it to communicate with an external SPI device, such as an EEPROM or DAC.

Code Example (MPLAB X + XC8):

#include <xc.h>

#define _XTAL_FREQ 8000000

void SPI_Init_Master(void)
{
    TRISC3 = 0;  // SCK as output
    TRISC4 = 1;  // SDI as input
    TRISC5 = 0;  // SDO as output
    TRISA5 = 0;  // SS as output
    
    SSPSTAT = 0x40;    // Transmit data on rising edge
    SSPCON1 = 0x21;    // Enable SPI, Master mode, Fosc/16
}

void SPI_Write(unsigned char data)
{
    SSPBUF = data;
    while(!BF);         // Wait until transmission complete
}

unsigned char SPI_Read(void)
{
    SSPBUF = 0x00;      // Send dummy data
    while(!BF);
    return SSPBUF;
}

#include <xc.h>

#define _XTAL_FREQ 8000000

void SPI_Init_Master(void)
{
    TRISC3 = 0;  // SCK as output
    TRISC4 = 1;  // SDI as input
    TRISC5 = 0;  // SDO as output
    TRISA5 = 0;  // SS as output
    
    SSPSTAT = 0x40;    // Transmit data on rising edge
    SSPCON1 = 0x21;    // Enable SPI, Master mode, Fosc/16
}

void SPI_Write(unsigned char data)
{
    SSPBUF = data;
    while(!BF);         // Wait until transmission complete
}

unsigned char SPI_Read(void)
{
    SSPBUF = 0x00;      // Send dummy data
    while(!BF);
    return SSPBUF;
}

Step 2: Circuit Diagram

To visualize the setup, connect the PIC18F4550 as Master to an SPI Slave device like a 25LC256 EEPROM or MCP4921 DAC.

Connections:

• RC3 (SCK) → Slave SCK
• RC5 (SDO) → Slave SDI
• RC4 (SDI) → Slave SDO
• RA5 (SS) → Slave CS
• GND ↔ GND, VCC ↔ VCC

Would you like me to generate a SPI circuit diagram image (PIC18F4550 + 25LC256 EEPROM with all lines labeled)? I can prepare it next.

🔹 Step 3: Data Transmission Example

Example: Writing a byte (0x55) to an EEPROM.

PORTAbits.RA5 = 0;      // Select Slave
SPI_Write(0x55);        // Send data
PORTAbits.RA5 = 1;      // Deselect Slave

PORTAbits.RA5 = 0;      // Select Slave
SPI_Write(0x55);        // Send data
PORTAbits.RA5 = 1;      // Deselect Slave

Step 4: Testing SPI

You can test the SPI communication by reading the data back from the slave or by observing waveforms on an oscilloscope.

Real-World Applications

• Interfacing EEPROMs (25LC256, AT25 series)
• DACs and ADCs
• TFT displays (ILI9341, ST7735)
• Wireless modules (NRF24L01, RF Transceivers)

Learn More

Are you excited to build projects like SPI-based EEPROM storage, digital potentiometers, and display drivers?
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Software development tools & documents:

• MPLAB X IDE
• MPLAB X, XC8 Compiler
• Microchip Technology, pic18f4550

Tags: SPI in PIC (SPI Module of PIC18F4550), Serial Peripheral Interface

💬Frequently Asked Questions (FAQ)