UART Flow Control: Worked Example

Worked example

Worked Example for UART Flow Control focuses on overrun count, baud mismatch, interrupt service latency. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

A product workload shows overrun count, baud mismatch, interrupt service latency. 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 UART Flow Control.

Sequence under inspection

SEQUENCE — UART Flow Control

  initiator            interconnect/PHY            target
      |  request (id) ------->  |                     |
      |                         |  forward ----------> |
      |                         |                     | work
      |                         |  <---- response ---- |
      |  <----- complete ------ |                     |
      |
   metric captured here: overrun count, baud mismatch, interrupt service latency

UART frame

UART FRAME (8N1)

idle ‾‾‾‾\__ D0 D1 D2 D3 D4 D5 D6 D7 /‾‾‾ idle
         ^start          (LSB first)   ^stop
no shared clock: both sides must agree on baud within ~2-3%

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 RX/TX waveform, FIFO watermark log, driver ISR trace.

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

Did the fix work?

BEFORE / AFTER — UART Flow Control

           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 overrun count, baud mismatch, interrupt service latency drop 40% after a seemingly small change near UART Flow Control.

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