SPI with external devices (sensors, displays) in Embedded C - Time & Space Complexity

Time Complexity: SPI with external devices (sensors, displays)

O(n)

Understanding Time Complexity

When using SPI to talk with sensors or displays, it's important to know how long communication takes as data size grows.

We want to see how the time to send or receive data changes when we handle more bytes.

Scenario Under Consideration

Analyze the time complexity of the following SPI data transfer code.

void spi_transfer(uint8_t* data, uint16_t length) {
    for (uint16_t i = 0; i < length; i++) {
        SPI_SendByte(data[i]);
        while (!SPI_TransferComplete());
    }
}
    

This code sends each byte one by one over SPI and waits for the transfer to finish before sending the next.

Identify Repeating Operations

Look at what repeats in the code.

• Primary operation: Sending a byte over SPI and waiting for it to finish.
• How many times: This happens once for each byte in the data array, so exactly length times.

How Execution Grows With Input

As you send more bytes, the total time grows in a straight line with the number of bytes.

Input Size (n)	Approx. Operations
10	10 byte sends and waits
100	100 byte sends and waits
1000	1000 byte sends and waits

Pattern observation: Doubling the number of bytes doubles the total time needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to send data grows directly with the number of bytes you send.

Common Mistake

[X] Wrong: "Sending more bytes over SPI takes the same time as sending just one byte."

[OK] Correct: Each byte requires its own send and wait, so more bytes mean more total time.

Interview Connect

Understanding how SPI communication time grows helps you design efficient embedded systems and answer questions about data transfer speed.

Self-Check

"What if the SPI transfer was done using DMA (Direct Memory Access) instead of waiting for each byte? How would the time complexity change?"

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