Clock polarity and phase (CPOL, CPHA) in Embedded C

Introduction

Clock polarity and phase settings control how data is sent and received in SPI communication. They make sure both devices agree on when to read and write bits.

When connecting a microcontroller to a sensor that uses SPI communication.

When setting up communication between two devices that share a clock signal.

When troubleshooting data errors in SPI communication.

When configuring SPI settings for a new hardware module.

When you need to match the clock settings of an external device to communicate correctly.

Syntax

Embedded C

SPI_InitTypeDef SPI_InitStructure;

SPI_InitStructure.CPOL = SPI_CPOL_Low;  // Clock polarity: clock idle state low
SPI_InitStructure.CPHA = SPI_CPHA_1Edge; // Clock phase: data captured on first clock edge

CPOL sets the idle state of the clock line (0 = low, 1 = high).

CPHA sets which clock edge data is sampled on (first or second).

Examples

Clock is low when idle, data is sampled on the first clock edge.

Embedded C

SPI_InitStructure.CPOL = SPI_CPOL_Low;
SPI_InitStructure.CPHA = SPI_CPHA_1Edge;

Clock is high when idle, data is sampled on the second clock edge.

Embedded C

SPI_InitStructure.CPOL = SPI_CPOL_High;
SPI_InitStructure.CPHA = SPI_CPHA_2Edge;

Clock is low when idle, data is sampled on the second clock edge.

Embedded C

SPI_InitStructure.CPOL = SPI_CPOL_Low;
SPI_InitStructure.CPHA = SPI_CPHA_2Edge;

Clock is high when idle, data is sampled on the first clock edge.

Embedded C

SPI_InitStructure.CPOL = SPI_CPOL_High;
SPI_InitStructure.CPHA = SPI_CPHA_1Edge;

Sample Program

This program sets up SPI clock polarity and phase, then prints the settings. It shows how CPOL and CPHA control SPI timing.

Embedded C

#include <stdio.h>

// Simulated SPI configuration structure
typedef struct {
    int CPOL; // Clock polarity
    int CPHA; // Clock phase
} SPI_InitTypeDef;

// Constants for CPOL
#define SPI_CPOL_Low 0
#define SPI_CPOL_High 1

// Constants for CPHA
#define SPI_CPHA_1Edge 0
#define SPI_CPHA_2Edge 1

void SPI_Init(SPI_InitTypeDef* SPI_InitStruct) {
    printf("SPI Clock Polarity (CPOL): %s\n", SPI_InitStruct->CPOL == SPI_CPOL_Low ? "Low (0)" : "High (1)");
    printf("SPI Clock Phase (CPHA): %s\n", SPI_InitStruct->CPHA == SPI_CPHA_1Edge ? "1st Edge (0)" : "2nd Edge (1)");
}

int main() {
    SPI_InitTypeDef SPI_Config;

    // Set clock polarity to low (idle low)
    SPI_Config.CPOL = SPI_CPOL_Low;
    // Set clock phase to first edge
    SPI_Config.CPHA = SPI_CPHA_1Edge;

    SPI_Init(&SPI_Config);

    return 0;
}

Important Notes

CPOL and CPHA must match on both SPI devices to communicate correctly.

Changing CPOL changes the clock's idle state, while CPHA changes when data is sampled.

Incorrect CPOL/CPHA settings cause data errors or no communication.

Summary

CPOL sets the clock's idle level (low or high).

CPHA sets which clock edge data is read or written.

Both must be set correctly for SPI devices to talk properly.