SPI library usage in Arduino - Step-by-Step Execution

Concept Flow - SPI library usage

Include SPI library

↓

Initialize SPI settings

↓

Begin SPI communication

↓

Send/Receive data

↓

End SPI communication

This flow shows how to include the SPI library, set it up, start communication, transfer data, and then end communication.

Execution Sample

Arduino

  #include <SPI.h>

  void setup() {
    SPI.begin();
    SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
  }

  void loop() {
    byte response = SPI.transfer(0x42);
  }

This code initializes SPI, starts a transaction with specific settings, and sends a byte while receiving a response.

Execution Table

Step	Action	SPI State	Data Sent	Data Received
1	Include SPI library	Not started	N/A	N/A
2	Call SPI.begin()	SPI initialized	N/A	N/A
3	Call SPI.beginTransaction(4MHz, MSBFIRST, SPI_MODE0)	Transaction started	N/A	N/A
4	Call SPI.transfer(0x42)	Transaction active	0x42	0x00 (dummy)
5	Loop repeats SPI.transfer if called again	Transaction active	0x42	0x00 (dummy)

💡 SPI communication continues until program stops or SPI.endTransaction() is called

Variable Tracker

Variable	Start	After SPI.begin()	After SPI.beginTransaction()	After SPI.transfer()
SPI State	Not started	Initialized	Transaction active	Transaction active
Data Sent	N/A	N/A	N/A	0x42
Data Received	N/A	N/A	N/A	0x00 (dummy)

Key Moments - 3 Insights

Why do we call SPI.begin() before transferring data?

SPI.begin() sets up the hardware SPI pins and prepares the SPI bus; without it, SPI.transfer() won't work properly as shown in step 2 of the execution_table.

What does SPI.beginTransaction() do?

It sets the SPI speed, bit order, and mode for communication, ensuring devices talk correctly; see step 3 where transaction starts with specific settings.

Why does SPI.transfer() return a value?

SPI is full-duplex, so while sending data, it simultaneously receives data from the slave device; step 4 shows sending 0x42 and receiving 0x00 (dummy data).

Visual Quiz - 3 Questions

Look at the execution_table, what is the SPI state after calling SPI.begin()?

A Not started

B Transaction active

C Initialized

D Ended

💡 Hint

Check the SPI State column at step 2 in the execution_table

At which step does the SPI transaction start with specific settings?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for 'Transaction started' in the SPI State column in execution_table

If SPI.beginTransaction() was not called, what would likely happen when SPI.transfer() is called?

A Data would send and receive normally

B SPI.transfer() would fail or use default settings

C SPI bus would reset automatically

D SPI.transfer() would send zero bytes

💡 Hint

Refer to key_moments about the role of SPI.beginTransaction() and step 3 in execution_table

Concept Snapshot

SPI library usage in Arduino:
- Include <SPI.h>
- Call SPI.begin() to initialize SPI pins
- Use SPI.beginTransaction(SPISettings) to set speed, bit order, mode
- Use SPI.transfer(byte) to send and receive data
- End with SPI.endTransaction() when done

Full Transcript

This visual execution shows how to use the Arduino SPI library step-by-step. First, the SPI library is included. Then SPI.begin() initializes the SPI hardware pins. Next, SPI.beginTransaction() sets communication speed, bit order, and mode. After that, SPI.transfer() sends a byte and simultaneously receives a byte from the slave device. The SPI state and data sent/received are tracked at each step. Key moments explain why initialization and transactions are needed and why SPI.transfer returns data. The quiz tests understanding of SPI states and transaction steps. The snapshot summarizes the main SPI usage steps in Arduino.

Practice

1. What is the main purpose of the SPI.begin() function in Arduino's SPI library?

easy

A. To close the SPI communication

B. To initialize the SPI bus and prepare it for communication

C. To set the SPI clock speed

D. To send a byte of data over SPI

Solution

1. Step 1: Understand the role of SPI.begin()

   The SPI.begin() function sets up the SPI hardware and pins for communication.

2. Step 2: Differentiate from other SPI functions

   Functions like SPI.transfer() send data, while SPI.beginTransaction() sets speed and mode, but SPI.begin() only initializes.

3. Final Answer:

   To initialize the SPI bus and prepare it for communication -> Option B

4. Quick Check:

   SPI.begin() initializes SPI [OK]

Hint: Remember: begin() always starts or initializes [OK]

Common Mistakes:

• Confusing SPI.begin() with SPI.transfer()
• Thinking SPI.begin() sets speed or mode
• Assuming SPI.begin() closes communication

2. Which of the following is the correct syntax to send a byte of data using the SPI library in Arduino?

easy

A. SPI.send(0xFF);

B. SPI.sendByte(0xFF);

C. SPI.write(0xFF);

D. SPI.transfer(0xFF);

Solution

1. Step 1: Identify the correct SPI function for sending data

   The Arduino SPI library uses SPI.transfer() to send and receive a byte simultaneously.

2. Step 2: Check syntax correctness

   Only SPI.transfer(0xFF); matches the correct function and parameter format.

3. Final Answer:

   SPI.transfer(0xFF); -> Option D

4. Quick Check:

   SPI.transfer() sends data byte [OK]

Hint: Use transfer() to send and receive SPI bytes [OK]

Common Mistakes:

• Using non-existent functions like send() or sendByte()
• Confusing SPI.transfer() with SPI.begin()
• Using write() which is not part of SPI library

3. What will be the output of this Arduino code snippet using SPI?

SPI.begin();
byte response = SPI.transfer(0xAA);
Serial.println(response, HEX);

Assuming the SPI slave device echoes back the sent byte.

medium

A. AA

B. 55

C. 00

D. FF

Solution

1. Step 1: Understand SPI.transfer() behavior

   The SPI.transfer() sends a byte and simultaneously receives a byte from the slave device.

2. Step 2: Analyze the slave device response

   The slave echoes back the sent byte 0xAA, so response will be 0xAA.

3. Final Answer:

   AA -> Option A

4. Quick Check:

   Sent byte echoed back = AA [OK]

Hint: SPI.transfer() returns the received byte immediately [OK]

Common Mistakes:

• Assuming transfer() only sends without receiving
• Expecting a different response byte
• Confusing HEX output with decimal

4. Identify the error in this Arduino SPI code snippet:

SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
SPI.transfer(0x01);
SPI.endTransaction();
SPI.begin();

medium

A. SPI.begin() should be called before SPI.beginTransaction()

B. SPI.endTransaction() is missing

C. SPI.transfer() cannot be used inside a transaction

D. SPISettings parameters are incorrect

Solution

1. Step 1: Check SPI initialization order

   SPI.begin() must be called once before starting any transaction to initialize SPI hardware.

2. Step 2: Verify transaction usage

   The code calls SPI.beginTransaction() before SPI.begin(), which is incorrect and causes errors.

3. Final Answer:

   SPI.begin() should be called before SPI.beginTransaction() -> Option A

4. Quick Check:

   Initialize SPI first with begin() [OK]

Hint: Always call SPI.begin() before transactions [OK]

Common Mistakes:

• Calling beginTransaction() before begin()
• Forgetting to call endTransaction()
• Misunderstanding SPISettings parameters

5. You want to communicate with two SPI devices on the same Arduino. How should you manage SPI transactions to avoid conflicts?

hard

A. Use SPI.transfer() without transactions for both devices

B. Call SPI.begin() twice, once for each device

C. Call SPI.beginTransaction() with each device's settings before communication and SPI.endTransaction() after

D. Connect both devices to the same chip select pin

Solution

1. Step 1: Understand SPI multi-device communication

   Each SPI device may require different speed or mode settings, so transactions help manage these safely.

2. Step 2: Use transactions properly

   Call SPI.beginTransaction() with the device's settings before communicating, then SPI.endTransaction() after to avoid conflicts.

3. Final Answer:

   Call SPI.beginTransaction() with each device's settings before communication and SPI.endTransaction() after -> Option C

4. Quick Check:

   Use transactions per device to avoid conflicts [OK]

Hint: Use beginTransaction/endTransaction per device [OK]

Common Mistakes:

• Calling SPI.begin() multiple times
• Sharing chip select pins for different devices
• Skipping transactions causing wrong settings