SPI Modes & Chip-Select Rules: Worked Example

Worked example

Worked Example for SPI Modes & Chip-Select Rules focuses on frame error rate, chip-select timing violation, throughput. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

A product workload shows frame error rate, chip-select timing violation, throughput. The first review mistake is to blame the whole interface. A better review starts by pinning one transaction, proving where protocol progress stopped, and checking whether the observed behavior is legal for SPI Modes & Chip-Select Rules.

Sequence under inspection

SEQUENCE — SPI Modes & Chip-Select Rules

  initiator            interconnect/PHY            target
      |  request (id) ------->  |                     |
      |                         |  forward ----------> |
      |                         |                     | work
      |                         |  <---- response ---- |
      |  <----- complete ------ |                     |
      |
   metric captured here: frame error rate, chip-select timing violation, throughput

SPI modes (CPOL/CPHA)

SPI MODES

Mode | CPOL | CPHA | sample edge
-----+------+------+-------------
  0  |  0   |  0   | rising
  1  |  0   |  1   | falling
  2  |  1   |  0   | falling
  3  |  1   |  1   | rising

WAVE (Mode 0):
CS   ‾‾\____________________/‾‾
SCLK ____|‾|_|‾|_|‾|_|‾|________
MOSI    D7  D6  D5  D4
        ^ sampled on rising edge

1. Capture the failing waveform and transaction log.

2. Tag the request ID, address, endpoint, or lane.

3. Find the first response, retry, stall, or missing completion.

4. Compare against MOSI/MISO/SCLK/CS waveform, mode table, device timing spec.

5. Choose one reversible fix and write the regression list before editing RTL or firmware.

Did the fix work?

BEFORE / AFTER — SPI Modes & Chip-Select Rules

           failing        target
metric  |    ●              ┄┄┄┄┄┄┄
        |     \
        |      \___ ● bounded fix
        |           \
        |            ● validated
        +-------------------------------> change set
Prove the mechanism moved the metric; one good dot is not proof.

Protocol deep dive

I2C/SPI/UART bugs are contract bugs: timing, reset value, IRQ type, and DMA watermark.

Concept diagram

PERIPHERAL CONTRACT

firmware writes regs -> RTL state machine -> pins -> board -> device
        ^                    |
        +------- IRQ/DMA ----+

If IRQ is level but driver assumes edge, you get lost events.

Metric graph

FIFO WATERMARK vs DMA

FIFO fill
 100%|        *** overrun risk
  75%|     ***
  50%|  ***     <- ideal DMA trigger band
  25%| *
   0%+----------------> time

Metrics and artifacts to collect

• NACK rate

• overrun count

• CS setup/hold violations

• IRQ miss rate

Mini case study

SPI flash worked in loopback but failed in system: CS deasserted one cycle early relative to device hold time. Board + RTL + mode bits together formed the contract.

Debug branches

• If overrun, FIFO depth vs ISR latency vs DMA burst.

• If NACK on I2C, pull-ups, speed, and clock stretch.

• If garbage data, CPOL/CPHA and MSB/LSB first.

Senior review question

Ask: what is the first transaction that deviates, and which spec rule does it test?

Key takeaways

• Connect every protocol claim to a transaction identity and measurable metric.

• Store the artifact (waveform, log, counter) next to every signoff decision.

Common pitfalls

• Debugging timeouts without finding the first bad transaction.

• Quoting peak bus width without payload efficiency and retry overhead.

• Treating VIP compliance as a substitute for system integration replay.

Narrative walkthrough

A team sees frame error rate, chip-select timing violation, throughput drop 40% after a seemingly small change near SPI Modes & Chip-Select Rules.

They almost widen the interface. Instead they capture id=7 read burst and find W beats never matched AW len.