Chip select management in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this chip select toggle code?

Consider the following embedded C code snippet managing a chip select pin. What will be the final state of the CS_PIN after execution?

Embedded C

#define CS_PIN 5
#define HIGH 1
#define LOW 0

int chip_select_state = HIGH;

void toggle_cs() {
    chip_select_state = LOW;
    // some SPI communication here
    chip_select_state = HIGH;
}

int main() {
    toggle_cs();
    return chip_select_state;
}

A 5

B 0

C 1

D Undefined behavior

💡 Hint

Think about the last assignment to chip_select_state inside toggle_cs().

The chip_select_state is set to LOW first, then back to HIGH before the function ends. So the final returned value is HIGH which is 1.

🧠 Conceptual

intermediate

Which statement best describes chip select (CS) behavior in SPI communication?

In SPI communication, what is the main purpose of the chip select (CS) line?

A It resets the SPI bus after each transfer.

B It carries the data from master to slave.

C It provides the clock signal for data transfer.

D It selects which slave device the master communicates with by enabling it.

💡 Hint

Think about how multiple devices share the SPI bus.

The chip select line is used to enable a specific slave device so it listens to the master. It does not carry clock or data signals.

🔧 Debug

advanced

Why does this chip select code cause a stuck SPI device?

Examine the code below. Why might the SPI slave device remain selected indefinitely?

Embedded C

void spi_transfer() {
    CS_PIN = 0; // activate chip select
    // send data over SPI
    // missing CS_PIN reset
}

int main() {
    spi_transfer();
    // other code
    return 0;
}

A CS_PIN is never set back to 1, so the device stays selected.

B The function spi_transfer() is missing a return statement.

C CS_PIN should be set to 2 to deselect the device.

D SPI data is not sent because CS_PIN is set to 0.

💡 Hint

Think about what happens if chip select stays low after communication.

If the chip select pin is not set back to high, the slave device remains active and may block further SPI communication.

📝 Syntax

advanced

Which option correctly toggles chip select in embedded C?

Choose the code snippet that correctly toggles the chip select pin CS_PIN from high to low and back to high.

A

CS_PIN := 0;
// SPI transfer
CS_PIN := 1;

B

CS_PIN = 0;
// SPI transfer
CS_PIN = 1;

C

CS_PIN = LOW;
// SPI transfer
CS_PIN = HIGH;

D

CS_PIN == 0;
// SPI transfer
CS_PIN == 1;

💡 Hint

Remember the assignment operator in C.

Option B uses the correct assignment operator = and proper semicolons. Option B uses invalid := syntax, Option B uses comparison ==, C misses semicolons.

🚀 Application

expert

How many times is the chip select toggled in this SPI burst?

Given the code below, how many times does the chip select pin toggle (go low then high) during the send_burst() function?

Embedded C

#define CS_PIN 10
#define LOW 0
#define HIGH 1

void send_byte(char b) {
    CS_PIN = LOW;
    // send byte over SPI
    CS_PIN = HIGH;
}

void send_burst(char *data, int length) {
    for (int i = 0; i < length; i++) {
        send_byte(data[i]);
    }
}

int main() {
    char message[3] = {0xA1, 0xB2, 0xC3};
    send_burst(message, 3);
    return 0;
}

A 3

B 1

C 6

D 0

💡 Hint

Count how many times send_byte is called and how many toggles happen per call.

The chip select toggles once per byte sent. Since 3 bytes are sent, it toggles 3 times.