SPI master-slave architecture in Embedded C

Introduction

SPI master-slave architecture helps devices talk to each other quickly using a simple wire connection. One device controls the conversation (master), and others listen and respond (slaves).

When connecting a microcontroller to sensors or memory chips.

When you want fast data transfer between two devices.

When you need to control multiple devices with one controller.

When you want simple wiring with clear control signals.

Syntax

Embedded C

/* SPI Master Initialization Example */
void SPI_MasterInit(void) {
    /* Set MOSI, SCK as Output, MISO as Input */
    DDRB = (1 << DDB3) | (1 << DDB5);
    /* Enable SPI, Set as Master, Set clock rate fck/16 */
    SPCR = (1 << SPE) | (1 << MSTR) | (1 << SPR0);
}

/* SPI Slave Initialization Example */
void SPI_SlaveInit(void) {
    /* Set MISO as Output */
    DDRB = (1 << DDB4);
    /* Enable SPI */
    SPCR = (1 << SPE);
}

Master controls clock and data flow.

Slave waits for master's clock to send/receive data.

Examples

Master writes data and waits for it to send.

Embedded C

/* Master sends a byte */
void SPI_MasterTransmit(char data) {
    SPDR = data; /* Load data into SPI data register */
    while(!(SPSR & (1 << SPIF))) ; /* Wait until transmission complete */
}

Slave waits for data from master and reads it.

Embedded C

/* Slave receives a byte */
char SPI_SlaveReceive(void) {
    while(!(SPSR & (1 << SPIF))) ; /* Wait for data reception */
    return SPDR; /* Return received data */
}

Sample Program

This program sets up a device as SPI master and sends the letter 'A' repeatedly to a slave device.

Embedded C

#include <avr/io.h>

void SPI_MasterInit(void) {
    DDRB = (1 << DDB3) | (1 << DDB5) | (1 << DDB2); /* MOSI, SCK, SS as output */
    SPCR = (1 << SPE) | (1 << MSTR) | (1 << SPR0); /* Enable SPI, Master, clk/16 */
}

void SPI_MasterTransmit(char data) {
    SPDR = data;
    while(!(SPSR & (1 << SPIF))) ;
}

int main(void) {
    SPI_MasterInit();
    while(1) {
        SPI_MasterTransmit('A'); /* Send character 'A' */
        for (volatile int i = 0; i < 10000; i++); /* Simple delay */
    }
    return 0;
}

Important Notes

Always set the slave select (SS) pin correctly to choose which slave to talk to.

SPI is full-duplex: master and slave send data at the same time.

Clock polarity and phase must match on master and slave for correct communication.

Summary

SPI master controls the clock and starts communication.

Slaves respond when selected by the master.

SPI is fast and uses few wires for device communication.

Practice

1. In SPI master-slave architecture, who controls the clock signal?

easy

A. The master device controls the clock signal.

B. The slave device controls the clock signal.

C. Both master and slave share clock control equally.

D. No clock signal is used in SPI communication.

Solution

1. Step 1: Understand SPI roles

   In SPI, the master initiates communication and controls timing.

2. Step 2: Identify clock control

   The master generates the clock signal to synchronize data transfer.

3. Final Answer:

   The master device controls the clock signal. -> Option A

4. Quick Check:

   Master controls clock = A [OK]

Hint: Master always controls clock in SPI [OK]

Common Mistakes:

• Thinking slave controls clock
• Assuming no clock is needed
• Believing clock is shared equally

2. Which of the following is the correct way to select a slave device in SPI using embedded C?

easy

A. Send a special data byte to select the slave.

B. Set the slave select (SS) pin HIGH to select the slave.

C. Toggle the clock pin to select the slave.

D. Set the slave select (SS) pin LOW to select the slave.

Solution

1. Step 1: Recall SPI slave selection method

   In SPI, the master selects a slave by pulling its SS pin LOW.

2. Step 2: Understand pin logic

   Setting SS LOW activates the slave; HIGH deselects it.

3. Final Answer:

   Set the slave select (SS) pin LOW to select the slave. -> Option D

4. Quick Check:

   SS LOW selects slave = A [OK]

Hint: SS pin LOW means slave selected [OK]

Common Mistakes:

• Setting SS HIGH to select slave
• Using clock pin to select slave
• Sending data to select slave

3. Given this embedded C code snippet for SPI master:

SPI_SS_PIN = 0; // Select slave
SPI_Write(0xA5); // Send data
SPI_SS_PIN = 1; // Deselect slave

What happens on the SPI bus during this code execution?

medium

A. The master selects the slave, sends 0xA5, then deselects the slave.

B. The slave sends 0xA5 to the master.

C. The master sends 0xA5 without selecting any slave.

D. The slave select pin stays HIGH, so no communication occurs.

Solution

1. Step 1: Analyze slave select pin changes

   Setting SPI_SS_PIN to 0 selects the slave before sending data.

2. Step 2: Understand data transmission

   SPI_Write(0xA5) sends data from master to selected slave.

3. Step 3: Deselect slave after sending

   Setting SPI_SS_PIN to 1 ends communication by deselecting slave.

4. Final Answer:

   The master selects the slave, sends 0xA5, then deselects the slave. -> Option A

5. Quick Check:

   SS LOW + send data + SS HIGH = D [OK]

Hint: SS LOW before send, HIGH after send [OK]

Common Mistakes:

• Assuming slave sends data
• Ignoring slave select pin state
• Thinking data sends without slave selected

4. Identify the error in this SPI master code snippet:

SPI_SS_PIN = 1; // Deselect slave
SPI_Write(0x3C); // Send data
SPI_SS_PIN = 0; // Select slave

medium

A. Data 0x3C is invalid for SPI communication.

B. Slave select pin is toggled correctly; no error.

C. Slave is deselected before sending data, so data won't be received.

D. SPI_Write function should be called after deselecting slave.

Solution

1. Step 1: Check slave select pin timing

   Slave is deselected (SS=1) before sending data, so slave ignores data.

2. Step 2: Correct order for SPI communication

   Slave must be selected (SS=0) before sending data to receive it.

3. Final Answer:

   Slave is deselected before sending data, so data won't be received. -> Option C

4. Quick Check:

   SS must be LOW before send = C [OK]

Hint: Select slave before sending data [OK]

Common Mistakes:

• Sending data before selecting slave
• Assuming data sends regardless of SS pin
• Misunderstanding SPI_Write timing

5. You want to communicate with two SPI slaves using one master. Which approach correctly manages slave selection in embedded C?

hard

A. Connect both slaves to the same slave select pin and set it LOW to select both.

B. Use separate slave select pins for each slave and set only one LOW at a time.

C. Toggle the clock pin rapidly to switch between slaves.

D. Send a unique data byte to select each slave without changing slave select pins.

Solution

1. Step 1: Understand multiple slave selection

   Each slave needs its own slave select pin controlled by the master.

2. Step 2: Select only one slave at a time

   Set only one slave select pin LOW to activate that slave; others stay HIGH.

3. Step 3: Avoid simultaneous selection

   Selecting multiple slaves at once causes bus conflicts and data errors.

4. Final Answer:

   Use separate slave select pins for each slave and set only one LOW at a time. -> Option B

5. Quick Check:

   Separate SS pins, one LOW at a time = B [OK]

Hint: One SS pin LOW per slave communication [OK]

Common Mistakes:

• Using one SS pin for all slaves
• Trying to select slaves by clock toggling
• Selecting multiple slaves simultaneously