SPI data transfer sequence in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

SPI Data Transfer: What is the output?

Consider this SPI transfer code snippet. What will be the value of received_data after the transfer completes?

Embedded C

#define SPI_DATA_REGISTER (*(volatile unsigned char*)0x4001300C)
#define SPI_STATUS_REGISTER (*(volatile unsigned char*)0x40013008)
#define SPI_STATUS_TXE (1 << 1)  // Transmit buffer empty
#define SPI_STATUS_RXNE (1 << 0) // Receive buffer not empty

unsigned char spi_transfer(unsigned char data) {
    SPI_DATA_REGISTER = data; // Start transmission
    while (!(SPI_STATUS_REGISTER & SPI_STATUS_TXE)) {}
    while (!(SPI_STATUS_REGISTER & SPI_STATUS_RXNE)) {}
    return SPI_DATA_REGISTER; // Read received data
}

int main() {
    unsigned char sent_data = 0xA5;
    unsigned char received_data = spi_transfer(sent_data);
    return received_data;
}

A 0x5A

B 0xA5

C 0xFF

D 0x00

💡 Hint

Think about what happens in SPI: data sent is simultaneously received.

In SPI, when you send a byte, you simultaneously receive a byte. Here, the code writes 0xA5 and waits for transmission and reception to complete, then reads the received data. Since no other device modifies the data, the received byte matches the sent byte 0xA5.

🧠 Conceptual

intermediate

SPI Transfer Sequence: Which step comes first?

In an SPI data transfer sequence, which step must happen first to start communication?

A Set the chip select (CS) line low to enable the slave device

B Send the data byte to the SPI data register

C Wait for the receive buffer to be not empty

D Read the received data from the SPI data register

💡 Hint

Think about how the slave device knows communication is starting.

The chip select (CS) line must be set low first to enable the slave device. Only then can data be sent and received. Without enabling the slave, the SPI transfer won't start properly.

❓ Predict Output

advanced

SPI Transfer Loop: What is the final value?

Given this code that sends multiple bytes over SPI, what is the final value of last_received?

Embedded C

#define SPI_DATA_REG (*(volatile unsigned char*)0x4001300C)
#define SPI_STATUS_REG (*(volatile unsigned char*)0x40013008)
#define SPI_STATUS_TXE (1 << 1)
#define SPI_STATUS_RXNE (1 << 0)

unsigned char spi_transfer(unsigned char data) {
    SPI_DATA_REG = data;
    while (!(SPI_STATUS_REG & SPI_STATUS_TXE)) {}
    while (!(SPI_STATUS_REG & SPI_STATUS_RXNE)) {}
    return SPI_DATA_REG;
}

int main() {
    unsigned char data_to_send[3] = {0x10, 0x20, 0x30};
    unsigned char last_received = 0;
    for (int i = 0; i < 3; i++) {
        last_received = spi_transfer(data_to_send[i]);
    }
    return last_received;
}

A 0x30

B 0x10

C 0x00

D 0x60

💡 Hint

The last received byte matches the last sent byte in this simple loop.

The loop sends 0x10, then 0x20, then 0x30. Each call to spi_transfer returns the received byte, which matches the sent byte here. The final received byte is 0x30.

🔧 Debug

advanced

Identify the error in SPI transfer code

This SPI transfer function is intended to send and receive a byte. What error will it cause?

Embedded C

unsigned char spi_transfer(unsigned char data) {
    SPI_DATA_REGISTER = data;
    while (!(SPI_STATUS_REGISTER & SPI_STATUS_TXE)) {}
    while (!(SPI_STATUS_REGISTER & SPI_STATUS_RXNE)) {}
    return SPI_DATA_REGISTER;
}

A It causes a syntax error due to missing semicolon

B It returns the wrong register value

C It causes an infinite loop waiting for receive buffer flag

D It reads the data before transmission starts

💡 Hint

Check the flags being waited on in the while loops.

The code waits twice for the transmit buffer empty flag (TXE). It never waits for the receive buffer not empty flag (RXNE). This causes the second loop to wait forever if RXNE is never set.

🧠 Conceptual

expert

SPI Full-Duplex Transfer: What happens during data exchange?

In SPI full-duplex communication, what happens when the master sends a byte to the slave?

A The master only sends data; the slave sends data separately later

B The master sends a byte first, then waits to receive a byte after transmission

C The master sends a byte and the slave ignores it until the next clock cycle

D The master sends a byte and simultaneously receives a byte from the slave

💡 Hint

SPI is known for simultaneous send and receive.

SPI is a full-duplex protocol, meaning data is sent and received at the same time. When the master sends a byte, it also receives a byte simultaneously from the slave.