SPI with external devices (sensors, displays) in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

SPI Data Transmission Output

What will be the output on the SPI data register (SPDR) after executing this code snippet?

Embedded C

#define SPI_DATA_REG (*(volatile unsigned char*)0x2F)
#define SPI_STATUS_REG (*(volatile unsigned char*)0x2E)
#define SPI_ENABLE 0x40
#define SPI_INT_FLAG 0x80

void spi_init() {
    SPI_STATUS_REG = SPI_ENABLE;
}

void spi_send(unsigned char data) {
    SPI_DATA_REG = data;
    while (!(SPI_STATUS_REG & SPI_INT_FLAG));
}

int main() {
    spi_init();
    spi_send(0xA5);
    return SPI_DATA_REG;
}

A 0xA5

B 0x00

C 0xFF

D Program hangs in while loop

💡 Hint

Think about what value is written to the SPI data register and when the function returns.

The function spi_send writes 0xA5 to SPI_DATA_REG and waits for SPI_INT_FLAG to be set. After that, the data register still holds 0xA5, so main returns 0xA5.

🧠 Conceptual

intermediate

SPI Clock Polarity and Phase

Which SPI mode configuration correctly sets clock polarity (CPOL) to 1 and clock phase (CPHA) to 0?

A CPOL=0, CPHA=1 means clock idles high, data sampled on leading edge

B CPOL=1, CPHA=0 means clock idles low, data sampled on trailing edge

C CPOL=1, CPHA=0 means clock idles high, data sampled on leading edge

D CPOL=0, CPHA=0 means clock idles high, data sampled on trailing edge

💡 Hint

Remember CPOL controls idle clock level, CPHA controls sampling edge.

CPOL=1 means clock line is high when idle. CPHA=0 means data is sampled on the first (leading) clock edge after the clock starts.

🔧 Debug

advanced

SPI Read from Sensor Fails

This code tries to read a byte from an SPI sensor but always returns 0x00. What is the most likely cause?

Embedded C

unsigned char spi_read() {
    SPI_DATA_REG = 0xFF; // send dummy byte
    while (!(SPI_STATUS_REG & SPI_INT_FLAG));
    return SPI_DATA_REG;
}

int main() {
    spi_init();
    unsigned char val = spi_read();
    return val;
}

A SPI_STATUS_REG is incorrectly masked, causing infinite loop

B SPI clock not enabled, so no data is clocked in

C SPI_INT_FLAG is never set because SPI is in slave mode

D SPI_DATA_REG should not be written before reading

💡 Hint

Check if SPI clock is active to shift data in and out.

If SPI clock is not enabled or not running, no data is shifted in, so SPI_INT_FLAG never sets and data register remains 0x00.

📝 Syntax

advanced

Correct SPI Interrupt Handler Syntax

Which option shows the correct syntax for an SPI interrupt service routine (ISR) in embedded C for AVR microcontrollers?

A ISR(SPI_STC_vect) { /* handle SPI interrupt */ }

B void ISR_SPI_STC_vect(void) { /* handle SPI interrupt */ }

C interrupt SPI_STC_vect() { /* handle SPI interrupt */ }

D void SPI_STC_vect() { /* handle SPI interrupt */ }

💡 Hint

Look for the special ISR macro used in AVR GCC.

AVR GCC uses ISR(vector_name) macro to define interrupt handlers. Option A uses this correctly.

🚀 Application

expert

SPI Display Initialization Sequence

Given these SPI commands to initialize a display, what is the correct order to send them to properly initialize the device?

A 4,1,2,3

B 2,1,4,3

C 1,4,2,3

D 1,2,4,3

💡 Hint

Reset first, then wake up, set pixel format, then turn display on.

The display must be reset first (0x01), then woken from sleep (0x11), then pixel format set (0x3A), and finally display turned on (0x29).