Essential Skills for Embedded Development: Capturing SPI Signals with Logic Analyzers

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Basics of SPI Protocol

SPI (Serial Peripheral Interface) is a synchronous serial communication interface developed by Motorola, widely used in embedded systems to connect microcontrollers with various peripherals (such as Flash memory, sensors, display modules, etc.).

1.1 SPI Signal Lines

The SPI bus typically includes the following four signal lines:

• SCLK (Serial Clock): Clock signal generated by the master device
• MOSI (Master Out Slave In): Data output from the master device to the slave device
• MISO (Master In Slave Out): Data input to the master device from the slave device
• SS/CS (Slave Select/Chip Select): Slave device selection signal (active low)

1.2 SPI Operating Modes

SPI has four operating modes determined by clock polarity (CPOL) and clock phase (CPHA):

• Mode 0: CPOL=0, CPHA=0 (Clock idle is low, data is sampled on the first edge)
• Mode 1: CPOL=0, CPHA=1 (Clock idle is low, data is sampled on the second edge)
• Mode 2: CPOL=1, CPHA=0 (Clock idle is high, data is sampled on the first edge)
• Mode 3: CPOL=1, CPHA=1 (Clock idle is high, data is sampled on the second edge)

Preparing the Logic Analyzer

2.1 Choosing the Right Logic Analyzer

For SPI signal analysis, it is recommended to choose:

• At least a 4-channel logic analyzer (for SCLK, MOSI, MISO, CS)
• A sampling rate of at least 4 times the SPI clock frequency (recommended 8-10 times)
• Support for SPI protocol decoding functionality

Common choices include:

• Saleae Logic series
• DSLogic
• Kingst LA series
• Domestic options like Chipsee

2.2 Connecting the Logic Analyzer

1. Connect the logic analyzer to the computer via USB
2. Install the appropriate drivers and software
3. Use probes to connect to the SPI signal lines:

• Channel 0 → SCLK
• Channel 1 → MOSI
• Channel 2 → MISO
• Channel 3 → SS/CS

Ensure a good ground connection (connect the ground of the logic analyzer to the ground of the system under test)

Logic Analyzer Settings

Using Saleae Logic software as an example:

3.1 Basic Settings

1. Open the Logic software
2. Select the correct device
3. Set the sampling rate: Choose based on the SPI clock frequency, for example:

• For a 1MHz SPI clock, select a sampling rate of 8-10MHz

Set the sampling time: Choose based on the duration of communication, usually 1-10 seconds is sufficient

3.2 SPI Protocol Analyzer Settings

1. Click the “Analyzers” tab
2. Add the “SPI” analyzer
3. Configure channel mapping:

• Clock → Select the channel corresponding to SCLK
• MISO → Select the channel corresponding to MISO
• MOSI → Select the channel corresponding to MOSI
• Enable → Select the channel corresponding to CS

Set the SPI mode: Choose the correct CPOL and CPHA based on the target device

Set the bit order: Usually MSB First

Set the CS polarity: Usually Active Low

Capturing SPI Signals

4.1 Starting Capture

1. Ensure all connections are correct
2. Click the “Start” button to begin capture
3. Trigger the target device to perform SPI communication (e.g., execute an operation or run a test program)
4. After communication is complete, click the “Stop” button to end capture

4.2 Analyzing Capture Results

After capture is complete, the software will display:

• Raw waveforms: Showing the level changes of each signal line
• Decoded results: Displaying transmitted data in hexadecimal or binary
• Timing information: Showing the transmission time for each bit or byte

Typical SPI data frame display format:

Timestamp | MOSI Data | MISO Data
0.001s | 0x9F    | 0x00
0.002s | 0x00    | 0xEF
0.003s | 0x00    | 0x40
...

Common Issues and Solutions

5.1 No Signal or Incomplete Signal

Possible causes and solutions:

1. Poor probe contact: Reconnect the probe to ensure good contact
2. Ground not connected: Ensure the logic analyzer and the tested system share a common ground
3. Insufficient sampling rate: Increase the sampling rate
4. Improper trigger condition settings: Adjust the trigger conditions or use manual triggering

5.2 Poor Signal Quality

Possible causes and solutions:

1. Signal interference: Shorten probe wire length, use shielded cables
2. Insufficient signal amplitude: Check if the signal levels meet the logic analyzer’s requirements
3. Impedance mismatch: Consider impedance matching for long-distance transmission

5.3 Decoding Errors

Possible causes and solutions:

1. Incorrect SPI mode settings: Try different CPOL/CPHA combinations
2. Incorrect bit order settings: Try switching between MSB/LSB
3. Incorrect CS polarity settings: Try switching between Active High/Low
4. Clock phase offset: Adjust the clock sampling position (some advanced logic analyzers support this)

Advanced Techniques

6.1 Trigger Settings

Utilize advanced triggering features to capture specific SPI communications:

• Edge triggering: Trigger on the falling or rising edge of CS
• Pattern triggering: Trigger when a specific data pattern appears on MOSI
• Serial triggering: Trigger when a specific data sequence appears

6.2 Timing Analysis

Use the logic analyzer to measure:

• Clock frequency and duty cycle
• Setup time and hold time
• Byte/frame interval time
• Time from CS activation to the first clock edge

6.3 Long Duration Capture

For intermittent communication:

• Use segmented capture mode
• Set pre-trigger buffering
• Use external storage to save large amounts of data

Practical Cases

7.1 Case: Reading SPI Flash ID

1. Connect the logic analyzer to the SPI Flash circuit
2. Set the SPI mode to 0 (most Flash uses mode 0 or 3)
3. Capture the read ID command (usually 0x9F)
4. Analyze the returned manufacturer ID and device ID

Typical capture results:

MOSI: 0x9F 0x00 0x00 0x00
MISO: 0x00 0xEF 0x40 0x18

Indicating a manufacturer ID of 0xEF and a device ID of 0x4018

7.2 Case: Initializing SPI Display

1. Capture the display initialization sequence
2. Analyze the command and data format
3. Verify if the timing meets the specifications

Conclusion

Using a logic analyzer to capture SPI signals is an essential skill for developing and debugging embedded systems.

By correctly setting the logic analyzer parameters, properly connecting probes, and accurately interpreting capture results, one can effectively analyze SPI communication issues, verify protocol implementation correctness, and optimize communication performance. Mastering these techniques will greatly enhance the efficiency and quality of embedded development.

Remember, the key to successful SPI signal analysis lies in:

1. Correctly understanding the SPI protocol specifications
2. Selecting the appropriate logic analyzer and settings
3. Ensuring good physical connections
4. Patiently and meticulously analyzing capture results

With accumulated practical experience, you will be able to quickly diagnose various SPI communication issues and gain a deeper understanding of the interactions between devices.

– EOF –

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