SPI Signals That Lie

SPI Signals That Lie

SPI looks simple on paper: clock, data out, data in, chip select. Four wires, deterministic timing, done. In real projects, SPI failures often appear as “sometimes wrong bytes,” “first transfer fails,” or “only breaks on production boards.” These are the kind of bugs that waste days because the bus seems healthy at first glance.

The core lesson is that SPI integrity is not just protocol correctness. It is electrical timing, firmware sequencing, and peripheral state management combined.

Common failure classes:

  • clock polarity/phase mismatch masked by forgiving devices
  • chip-select timing violations near transaction boundaries
  • signal integrity problems at higher edge rates
  • peripheral state not reset between commands
  • DMA and interrupt races corrupting transfer order

Any one can produce plausible-but-wrong data.

I start with protocol truth first. Confirm CPOL/CPHA mode from datasheets, then verify with logic analyzer captures of command/response boundaries. Do not rely on “it worked with another sensor.” Different devices tolerate different mistakes.

Chip-select discipline is frequently underestimated. Some peripherals require minimum setup/hold time around CS transitions. If firmware toggles CS too quickly under optimization changes, a previously stable transfer can degrade silently. Enforce timing explicitly, not by incidental delays.

Signal integrity matters earlier than many assume. At modest board lengths and strong GPIO drive settings, ringing and overshoot can create false edges. Scope captures at the receiver pin, not just MCU pin, are essential. What leaves the MCU is not always what arrives at the device.

Practical board-level mitigations include:

  • series resistors near source on high-edge lines
  • clean return paths
  • reduced edge rate where available
  • controlled trace length matching for sensitive links

These are cheap changes with high payoff.

On firmware side, transaction framing should be explicit. Wrap transfers in one API that controls:

  • CS assert/deassert
  • mode and speed selection
  • optional guard delays
  • retry and timeout policy

Scattered raw register writes across drivers create hidden divergence and fragile maintenance.

DMA introduces its own failure modes. If buffer ownership and completion signaling are unclear, stale or partially updated data appears intermittently. Use strict ownership rules and assert expected transfer length at completion.

Interrupt interactions can also corrupt sequencing. If high-priority ISRs preempt between CS assert and first clock edge, timing contracts may break. Critical sections around transaction start are often justified in tight timing designs.

Another subtle trap: mixed-speed peripherals on shared bus. Reconfiguration bugs happen when one driver leaves bus speed or mode altered for the next device. Centralized bus arbitration prevents this class of bug.

Diagnostic strategy that works well:

  1. lock one known-good frequency and mode
  2. disable DMA and run blocking transfers
  3. validate deterministic test vectors
  4. reintroduce DMA and concurrency incrementally
  5. increase bus speed in controlled steps

When failures reappear, you know which complexity layer introduced them.

I strongly recommend adding protocol-level self-checks where possible:

  • read-back register after write
  • device ID verification at startup
  • command echo checks
  • CRC where supported

These catch latent bus corruption before higher-level logic misbehaves.

Power and reset sequencing also influence SPI reliability. Some peripherals accept clocks before internal state is ready, then remain in undefined mode until hard reset. Ensure boot initialization obeys datasheet timing windows.

For production robustness, perform variability tests:

  • temperature sweep
  • supply voltage corners
  • cable/harness variants where applicable
  • repeated long-run stress with error counters

If an SPI link passes only nominal lab conditions, it is not finished.

Logging can help in deployed systems:

  • transaction error counts
  • timeout counts
  • last failing opcode
  • bus-reset events

These metrics turn rare field failures into diagnosable patterns.

The big mindset shift: SPI bugs are often systems bugs, not line-by-line coding bugs. You solve them fastest by combining electrical captures, protocol verification, and firmware sequencing analysis, not by focusing on one layer alone.

If you keep one rule, keep this: trust captured timing and measured waveforms over assumptions. SPI rarely lies; our interpretation of partial evidence does.

If a design ships to production, add one recovery path too: a bus reinitialization routine that can safely reset peripheral state after repeated transaction failure. Rare field glitches become survivable when recovery is deterministic and observable rather than hidden behind random retries.

Design for recoverability, then verify it under stress.

2026-02-22