Use the selected serial/interrupt contract rather than naming a fault from a symptom alone.
Try these before opening the explanations. This is a self-assessment, not a stored score or certification.
No. The final read-byte NACK is the controller's intended end-of-read indication in this combined-transfer example. An address NACK occurs immediately after an address byte.
No: seven plus two is nine arrivals for eight slots. Use the worst-case delay, not the average. Controller acknowledgement also need not deassert the source: follow its documented W1C, drain or other condition-removal contract.
Ready to continue when you can produce:
Who drives the ninth I²C clock's acknowledgement at each transfer phase.
Worst-case receive headroom and the interrupt source's deassertion mechanism.
Confirm the ninth clock really contains a NACK, then check address/straps, power/reset/readiness, supported mode and line timing. An SCL-low clock-stretch timeout is a different observation and should be diagnosed separately.
Common trap: Calling any controller timeout a NACK or only checking the software address.
Wrong or shifted samples are possible, but the pattern is not unique. Compare CPOL/CPHA terminology, capture edges, word/bit order and chip-select timing with the device timing diagram and a bus capture.
Common trap: Diagnosing mode from one corrupted word or changing clock speed without checking edge semantics.
A character arrived when the receiver had no retention capacity. Measure FIFO/holding capacity, trigger level, frame rate, worst-case interrupt/DMA service latency, flow-control reaction and bytes already in flight. Baud mismatch is usually a separate framing/parity/data-error hypothesis.
Common trap: Treating every garbled byte as overrun or blaming only ISR speed.