SPI Communication Protocol

SPI Communication Protocol

Interactive timing lab for SPI chip select, SCLK, MOSI, MISO, CPOL, CPHA, and full-duplex byte transfers.

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A learner-ready SPI protocol animation with bit-by-bit waveform playback, mode controls, full-duplex shift-register view, chip-select fault checks, transfer-time math, quick reference, and technical accuracy notes.
Animation SPI Full Duplex Timing Modes

SPI Communication Protocol

Follow a controller and peripheral as chip select goes low, SCLK pulses, MOSI and MISO shift at the same time, and receivers sample on the edge defined by CPOL and CPHA.

Mode 0 Clock mode
b7: 1/0 Active bit
4.00 us Transfer time
Healthy transfer Bus status
TrySelect Sensor, choose Mode 0, set Clock rate to 1 MHz and Bit order to MSB first, then press Step.
ObserveThe Active bit, MOSI byte, MISO byte, and Transfer time move across 8 clock edges while Chip select remains asserted.
ExplainMode 0 samples on each rising edge and shifts on each falling edge, allowing both 8-bit registers to exchange one bit per clock in full duplex.
Technical boundariesWaveforms use ideal edges and one slave; setup and hold margins, propagation skew, level shifting, signal integrity, DMA gaps, multi-slave contention, CRC, and peripheral-specific framing are not modelled.
Colour keysensors identitycurrent / primaryreference / datasuccesscautionerror / failure
1. Select Only the target peripheral should have CS low.
2. Clock The controller owns SCLK and the selected SPI mode.
3. Shift MOSI and MISO move one bit each clock cycle.
4. Sample Receivers latch data on the selected sample edge.
5. Release CS returns high and inactive MISO drivers tri-state.

Bus and Timing Animation

The orange token follows the active bit. The green dot marks the sample edge for the selected CPOL and CPHA mode.

SPI controller connected to multiple peripherals SCLK MOSI MISO CS active low Controller drives SCLK MOSI: -- Sensor -- Display -- Flash --

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Transaction Controls

Choose a realistic SPI case, then adjust mode, clock rate, byte values, bit order, and chip-select behavior.

Only this device's CS line should be active low.
Mode sets CPOL, CPHA, and the sampling edge.
Higher clock rates shorten transfer time but stress layout and device timing.
Most sensor register protocols use MSB first.
Controller-to-peripheral byte.
Peripheral-to-controller byte shifted during the same clocks.
This animation displays one byte; transfer-time math can include two bytes.
Use faults to see why SPI failures can look like bad data.

Clock Mode

CPOL selects idle level. CPHA selects first or second edge.

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Transfer Time

One clock transfers one bit in each direction.

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Full Duplex

MOSI and MISO shift simultaneously.

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Chip Select Rule

Only the selected peripheral may drive MISO.

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SPI Quick Reference

Four common lines

  • SCLK: controller-generated clock.
  • MOSI: controller data to peripheral.
  • MISO: peripheral data to controller.
  • CS or SS: active-low device select.

Mode checklist

  • Mode 0: CPOL 0, CPHA 0.
  • Mode 1: CPOL 0, CPHA 1.
  • Mode 2: CPOL 1, CPHA 0.
  • Mode 3: CPOL 1, CPHA 1.

Debug order

  • Check CS polarity and target device.
  • Check mode and bit order against the datasheet.
  • Reduce SCLK before blaming parsing code.
  • Confirm inactive devices tri-state MISO.
Technical Accuracy Notes

No address or ACK layer

SPI does not include I2C-style addressing or ACK bits. Device selection is external through CS wiring, and data validity comes from the peripheral protocol.

Mode naming caveat

Most datasheets use CPOL and CPHA or mode numbers, but exact timing limits also include setup time, hold time, output delay, and maximum SCLK.

Physical limits

High-speed SPI is normally a board-level bus. Long cables, weak grounding, and multiple stubs can cause ringing and bit errors even when firmware settings are correct.

Practice Prompts

Find a mode fault

Run the sensor read preset, inject a mode mismatch, and explain why the sample edge is no longer valid for the peripheral.

Spot MISO contention

Inject two active CS lines. Explain why MOSI can still look normal while MISO becomes untrustworthy.

Budget transfer time

Switch from 8 bits to 16 bits and reduce SCLK. Estimate whether the transfer fits inside a short sensor sampling window.

I2C Protocol

Compare shared-address ACK behavior with SPI chip select.

I2C/SPI Visualizer

Compare two-wire and four-wire bus trade-offs.

ADC Sampling

See how SPI often moves sampled data out of converters.

Sensor Pipeline

Place the bus transaction inside the larger IoT data path.