SPI with external devices (sensors, displays) in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize SPI communication with the correct clock polarity.

Embedded C

SPI_InitTypeDef SPI_Config;
SPI_Config.CLKPolarity = [1];

Drag options to blanks, or click blank then click option'

ASPI_POLARITY_LOW

BSPI_POLARITY_HIGH

CSPI_MODE_MASTER

DSPI_FIRSTBIT_MSB

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing clock polarity with clock phase.

Using SPI_MODE_MASTER instead of polarity setting.

The clock polarity must be set to SPI_POLARITY_LOW to ensure the clock idles low, which is common for many SPI devices.

2fill in blank

medium

Complete the code to send a byte of data over SPI.

Embedded C

uint8_t data = 0xA5;
SPI_Transmit([1], 1);

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ANULL

B0xA5

C&data

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the data value directly instead of its address.

Passing NULL which causes no data to send.

The SPI transmit function requires a pointer to the data buffer, so &data is correct.

3fill in blank

hard

Fix the error in the SPI receive code to correctly read one byte.

Embedded C

uint8_t received;
SPI_Receive([1], 1);

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ANULL

Breceived

C*received

D&received

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable directly instead of its address.

Using NULL which causes no storage location.

The receive function needs a pointer to store the incoming data, so &received is correct.

4fill in blank

hard

Complete the sensor reading loop by filling both blanks to use the sensor address for SPI read and check if the value is above 100.

Embedded C

for (int i = 0; i < sensor_count; i++) {
    uint16_t value = SPI_Read([1]);
    if (value [2] 100) {
        sensor_values[i] = value;
    }
}

Drag options to blanks, or click blank then click option'

Asensor_addresses[i]

B>

C<

Dsensor_ids[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Using sensor IDs instead of addresses for SPI read.

Using less than instead of greater than in the condition.

The SPI_Read function should use the sensor's address, and the condition checks if the value is greater than 100.

5fill in blank

hard

Fill all three blanks to correctly configure SPI mode, data size, and first bit order.

Embedded C

SPI_Config.Mode = [1];
SPI_Config.DataSize = [2];
SPI_Config.FirstBit = [3];

Drag options to blanks, or click blank then click option'

ASPI_MODE_MASTER

BSPI_DATASIZE_8BIT

CSPI_FIRSTBIT_MSB

DSPI_MODE_SLAVE

Attempts:

3 left

💡 Hint

Common Mistakes

Setting slave mode when master is needed.

Using wrong data size constant.

Confusing first bit order.

To configure SPI as master with 8-bit data size and most significant bit first, use these constants.

Practice

1. What is the main purpose of the Chip Select (CS) line in SPI communication with external devices like sensors or displays?

easy

A. To increase the SPI clock speed

B. To provide power to the SPI device

C. To reset the SPI device

D. To select which device the microcontroller communicates with

Solution

1. Step 1: Understand SPI device selection

   The Chip Select (CS) line is used to enable communication with one specific SPI device among multiple devices connected to the same SPI bus.

2. Step 2: Identify the role of CS

   When CS is active (usually low), the selected device listens and responds to SPI signals; other devices ignore the communication.

3. Final Answer:

   To select which device the microcontroller communicates with -> Option D

4. Quick Check:

   CS line = device selector [OK]

Hint: CS line chooses the device to talk to in SPI [OK]

Common Mistakes:

• Confusing CS with power supply line
• Thinking CS resets the device
• Assuming CS controls clock speed

2. Which of the following is the correct way to initialize SPI communication in embedded C for a sensor device?

easy

A. SPI_Init(); CS_Pin = LOW; SPI_Transmit(0xAA);

B. SPI_Init(); CS_Pin = HIGH; SPI_Transmit(0xFF);

C. SPI_Start(); CS_Pin = LOW; SPI_Receive();

D. SPI_Begin(); CS_Pin = HIGH; SPI_Receive(0x00);

Solution

1. Step 1: Check SPI initialization and CS pin state

   SPI_Init() is the standard function to initialize SPI. CS must be LOW to select the device before transmitting data.

2. Step 2: Verify data transmission call

   SPI_Transmit(0xAA) sends data to the device. This matches typical SPI usage.

3. Final Answer:

   SPI_Init(); CS_Pin = LOW; SPI_Transmit(0xAA); -> Option A

4. Quick Check:

   Init SPI + CS low + transmit data = correct setup [OK]

Hint: CS LOW means device selected before sending data [OK]

Common Mistakes:

• Setting CS HIGH before transmit disables device
• Using wrong SPI init function name
• Calling receive without selecting device

3. Consider this embedded C code snippet communicating with an SPI sensor:

CS_PIN = 0;  // Select device
uint8_t data = SPI_TransmitReceive(0x55);
CS_PIN = 1;  // Deselect device

What value will data hold after execution?

medium

A. The sensor's response byte received during transmission

B. Always 0x55, the sent byte

C. Undefined, because CS_PIN is set to 1 after transmit

D. An error code due to missing SPI initialization

Solution

1. Step 1: Understand SPI_TransmitReceive behavior

   This function sends a byte (0x55) and simultaneously receives a byte from the sensor during the SPI clock cycles.

2. Step 2: Analyze CS pin usage

   CS_PIN is set LOW before transmission to select the device and set HIGH after to deselect. This is correct usage and does not affect the received data.

3. Final Answer:

   The sensor's response byte received during transmission -> Option A

4. Quick Check:

   SPI sends and receives simultaneously = sensor response [OK]

Hint: SPI sends and receives data at the same time [OK]

Common Mistakes:

• Assuming received data equals sent data
• Thinking CS high before transmit affects data
• Ignoring simultaneous send/receive nature

4. This code snippet is intended to read data from an SPI display, but it does not work correctly:

CS_PIN = 1;
SPI_Transmit(0x0F);
CS_PIN = 0;
uint8_t response = SPI_Receive();
CS_PIN = 1;

What is the main error?

medium

A. CS_PIN should remain HIGH during receive

B. SPI_Transmit should be SPI_Receive

C. CS_PIN should be set LOW before transmitting

D. SPI_Receive requires a parameter

Solution

1. Step 1: Check CS_PIN state before transmission

   CS_PIN must be LOW to select the SPI device before sending commands or data.

2. Step 2: Identify the error in CS_PIN usage

   The code sets CS_PIN HIGH before transmit, which deselects the device, so the device ignores the command.

3. Final Answer:

   CS_PIN should be set LOW before transmitting -> Option C

4. Quick Check:

   CS LOW before transmit = device listens [OK]

Hint: Always set CS LOW before sending SPI data [OK]

Common Mistakes:

• Setting CS HIGH before transmit disables device
• Confusing transmit and receive functions
• Assuming CS state doesn't affect communication

5. You want to read temperature data from an SPI sensor that requires sending a command byte 0x01, then reading two bytes of data. Which code snippet correctly implements this sequence?

hard

A. CS_PIN = 0;
SPI_Transmit(0x01);
uint8_t high = SPI_Receive();
uint8_t low = SPI_Receive();
CS_PIN = 1;

B. CS_PIN = 0;
SPI_Transmit(0x01);
high = SPI_Transmit(0x00);
low = SPI_Transmit(0x00);
CS_PIN = 1;

C. CS_PIN = 1;
SPI_Transmit(0x01);
high = SPI_Receive();
low = SPI_Receive();
CS_PIN = 0;

D. SPI_Transmit(0x01);
CS_PIN = 0;
high = SPI_Receive();
low = SPI_Receive();
CS_PIN = 1;

Solution

1. Step 1: Select device before communication

   CS_PIN must be set LOW before sending commands or data to select the sensor.

2. Step 2: Send command and read data correctly

   SPI_Transmit sends a byte and simultaneously receives a byte. To read data, send dummy bytes (0x00) to clock out sensor data.

3. Step 3: Deselect device after communication

   CS_PIN is set HIGH after communication to end the session.

4. Final Answer:

   CS_PIN = 0; SPI_Transmit(0x01); high = SPI_Transmit(0x00); low = SPI_Transmit(0x00); CS_PIN = 1; -> Option B

5. Quick Check:

   Send command, then send dummy bytes to receive data [OK]

Hint: Send dummy bytes to receive SPI data after command [OK]

Common Mistakes:

• Reading without sending dummy bytes
• Setting CS high before command
• Not deselecting device after communication