SPI with external devices (sensors, displays) in Embedded C - Step-by-Step Execution

Concept Flow - SPI with external devices (sensors, displays)

Start SPI Communication

↓

Set Chip Select LOW

↓

Send Data Byte

↓

Wait for Transmission Complete

↓

Read Received Data

↓

Set Chip Select HIGH

↓

End SPI Communication

This flow shows how SPI communication starts by selecting the device, sending and receiving data, then ending communication by deselecting the device.

Execution Sample

Embedded C

SPI_CS_LOW();
SPI_Transmit(0xA5);
while(!SPI_Transmission_Complete());
uint8_t data = SPI_Receive();
SPI_CS_HIGH();

This code selects the SPI device, sends a byte 0xA5, waits for transmission to complete, reads the received byte, then deselects the device.

Execution Table

Step	Action	SPI_CS State	Data Sent	Transmission Complete?	Data Received
1	Set Chip Select LOW	LOW	-	No	-
2	Send Data 0xA5	LOW	0xA5	No	-
3	Wait for Transmission Complete	LOW	0xA5	No	-
4	Transmission Complete	LOW	0xA5	Yes	-
5	Read Received Data	LOW	0xA5	Yes	0x3C
6	Set Chip Select HIGH	HIGH	0xA5	Yes	0x3C

💡 Chip Select set HIGH ends SPI communication with the device.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	After Step 5	Final
SPI_CS	HIGH	LOW	LOW	LOW	LOW	HIGH
Data Sent	-	-	0xA5	0xA5	0xA5	0xA5
Transmission Complete	No	No	No	Yes	Yes	Yes
Data Received	-	-	-	-	0x3C	0x3C

Key Moments - 3 Insights

Why do we set Chip Select LOW before sending data?

Setting Chip Select LOW tells the external device to listen and communicate. Without this, the device ignores SPI signals. See Step 1 in execution_table.

Why do we wait for transmission to complete before reading data?

SPI is synchronous; data is exchanged simultaneously. Waiting ensures the sent byte is fully transmitted and the received byte is ready. See Steps 3 and 4.

What happens if we don't set Chip Select HIGH after communication?

The device stays selected and may misinterpret further SPI signals. Setting it HIGH ends communication properly. See Step 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SPI_CS state after Step 3?

A Undefined

B HIGH

C LOW

D Toggling

💡 Hint

Check the SPI_CS State column at Step 3 in execution_table.

At which step does the transmission complete become 'Yes'?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the Transmission Complete? column in execution_table.

If we skip setting Chip Select HIGH at the end, what is the likely effect?

A Communication ends normally

B Device remains selected and may misbehave

C Data is lost immediately

D SPI bus resets automatically

💡 Hint

Refer to key_moments explanation about Chip Select behavior.

Concept Snapshot

SPI communication with devices:
1. Set Chip Select LOW to start.
2. Send and receive data byte.
3. Wait for transmission complete.
4. Read received data.
5. Set Chip Select HIGH to end communication.
Always control Chip Select to target the right device.

Full Transcript

This visual trace shows SPI communication steps with an external device like a sensor or display. First, the Chip Select line is set LOW to activate the device. Then a data byte (0xA5) is sent. The program waits until the transmission completes, ensuring data is fully sent and received. After that, the received data byte (0x3C) is read from the SPI buffer. Finally, Chip Select is set HIGH to end communication. Variables like SPI_CS state, data sent, transmission status, and data received are tracked step-by-step. Key points include why Chip Select must be controlled and why waiting for transmission completion is necessary. The quiz checks understanding of SPI_CS state, transmission timing, and Chip Select importance.

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