SPI with external devices (sensors, displays) in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SPI stand for and what is its main purpose?

SPI stands for Serial Peripheral Interface. It is a communication protocol used to send data between a microcontroller and external devices like sensors or displays quickly and efficiently.

beginner

Name the four main signals used in SPI communication.

The four main SPI signals are:
1. MOSI (Master Out Slave In) - data from master to slave
2. MISO (Master In Slave Out) - data from slave to master
3. SCLK (Serial Clock) - clock signal from master
4. SS (Slave Select) - selects which slave device to communicate with

beginner

How does the master device control communication in SPI?

The master device controls communication by generating the clock signal (SCLK) and selecting the slave device using the Slave Select (SS) line. It sends data on MOSI and reads data on MISO.

intermediate

Why is SPI preferred for connecting sensors and displays in embedded systems?

SPI is preferred because it is fast, simple, and supports full-duplex communication. It uses fewer wires than parallel interfaces and can connect multiple devices using separate SS lines.

beginner

What is the role of the Slave Select (SS) line in SPI communication?

The Slave Select (SS) line tells a specific slave device to listen and respond to the master. When SS is low (active), the slave is selected; when high, the slave ignores the SPI bus.

Which SPI signal carries data from the master to the slave?

A MOSI

B MISO

C SCLK

D SS

MOSI stands for Master Out Slave In, so it carries data from the master to the slave.

What does the master device generate in SPI communication?

A Only clock signal

B Only data signals

C Only Slave Select

D Clock signal and Slave Select

The master generates both the clock signal (SCLK) and the Slave Select (SS) to control communication.

In SPI, what happens when the Slave Select (SS) line is high?

A Slave device ignores communication

B Slave device is selected and active

C Master stops sending clock

D Data is sent on MISO

When SS is high, the slave device ignores the SPI bus and does not communicate.

Which of these is NOT a typical use of SPI?

A Connecting sensors

B Connecting displays

C Connecting to the internet

D Connecting memory chips

SPI is not used for internet connections; it is used for short-distance device communication like sensors and displays.

What type of communication does SPI support?

A Half-duplex only

B Full-duplex

C Simplex only

D No data transfer

SPI supports full-duplex communication, meaning data can be sent and received at the same time.

Explain how SPI communication works between a microcontroller and a sensor.

Think about who controls the clock and how data flows both ways.

Describe the role of each SPI signal line and why they are important.

Focus on what each wire does during communication.

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