FAQ
TL;DR: Intermittent AK8963C SPI reads traced to signal integrity: MISO sagged to 1.65 V while PIC VIH needs 2.3 V; “I suspect a timing issue.” Diagnosis with scope/analyzer and adding a buffer fixed most cases. [Elektroda, Anonymous, post #21683412]
Why it matters: This FAQ helps PIC users quickly debug flaky SPI compass readings using practical, proven steps.
For: PIC18F users integrating AK8963C over SPI who see random read failures or noisy waveforms.
Quick Facts
- PIC18F4620 input-high threshold noted as 2.3 V; below this, reads fail. [Elektroda, Anonymous, post #21683418]
- MISO intermittently fell to ~1.65 V, triggering misreads. [Elektroda, Anonymous, post #21683418]
- Approx. 200 mm wire run between PIC and compass increases skew risk. [Elektroda, Anonymous, post #21683414]
- AK8963C WHO_AM_I returned 0x48 during good reads. [Elektroda, Anonymous, post #21683414]
- SN74LV1T34 buffer suggested; 3.3 V VCC, with ~9 ns prop delay cited. [Elektroda, Anonymous, post #21683423]
How do I fix intermittent SPI reads from an AK8963C on a PIC18F?
Check MISO voltage first. If highs are ~1.65 V, they miss the PIC’s 2.3 V VIH. Add a unidirectional buffer like SN74LV1T34 at 3.3 V. Then slow SPI and verify CPOL/CPHA to meet setup/hold. Keep wires short and probe near the PIC. This sequence restored stable ID reads (0x48) for the thread author. [Elektroda, Anonymous, post #21683423]
Is this a timing or wiring problem?
Both can apply. Long leads (~200 mm) add delay and edge distortion. The expert noted, “I suspect a timing issue.” Slow the SPI clock, then check clock–data alignment on a two‑channel scope at the PIC pin. If alignment improves at lower speed, reduce cable length or add a buffer. [Elektroda, Anonymous, post #21683412]
What SPI edge should the PIC sample on here?
The setup sampled on the rising edge. Confirm your PIC’s SPI mode matches the AK8963C timing you observe. If data changes near the sampling edge, shift the phase or reduce speed to increase margin. Validate by reading WHO_AM_I = 0x48 consistently. [Elektroda, Anonymous, post #21683416]
Are 200 mm wires too long for reliable SPI?
They can be. The thread used ~200 mm between PIC and compass, which increases skew and ringing. Shorten runs, add series resistors near the driver, and maintain solid ground return. Re‑probe 30 mm from the PIC to verify improvement. [Elektroda, Anonymous, post #21683414]
Why does my MISO logic-high look low on the scope?
Loading or level mismatch. The helper observed MISO “less than 2 V,” which fails PIC VIH. Check for unintended pull‑downs or alternate pin functions on the PIC. Insert a buffer/level shifter to restore high level and edge rate. [Elektroda, Anonymous, post #21683417]
Will a MOSFET work as a level shifter on MISO?
A single N‑MOSFET wired low‑side inverts. Two stages re‑invert but add delay. The discussion favored a 74‑series buffer with a low VIH at 3.3 V. It avoids inversion and has ~9 ns delay, suitable for typical SPI speeds. [Elektroda, Anonymous, post #21683419]
Which single‑gate buffer was recommended?
SN74LV1T34. At 3.3 V supply, it accepts low input thresholds and provides clean 3.3 V CMOS outputs. The data point shared lists ~9 ns max propagation delay, which is acceptable for moderate SPI clocks and cleans up MISO transitions. [Elektroda, Anonymous, post #21683423]
Could the PIC pin be dragging MISO down?
Yes. The helper suspected PIC pin configuration loading the line. Check for enabled pull‑downs or alternate functions. Disable internal pulls on the MISO pin and retest. If the line rises with a buffer inserted, loading was likely. [Elektroda, Anonymous, post #21683426]
What’s the AK8963C device ID I should read over SPI?
WHO_AM_I returned 0x48 during correct operation in this setup. Use it as a quick health check after power‑up and after changing timing or wiring. If it flips or reads 0x00/0xFF, debug signal integrity first. [Elektroda, Anonymous, post #21683414]
Quick 3‑step: How do I isolate the failure fast?
- Scope CLK and MISO at the PIC pin; capture good vs bad reads.
- Slow SPI and shorten the cable or add a buffer.
- Verify WHO_AM_I = 0x48 repeatedly; then restore target SPI speed.
[Elektroda, Anonymous, post #21683412]
What is the AK8963C in this context?
It’s a compass/magnetometer module harvested from an iPhone 5, used here as an SPI peripheral to a PIC18F board. The project had several modules and PCBs, confirming the issue wasn’t a single bad unit. [Elektroda, Anonymous, post #21683408]
What’s an edge‑case failure to watch for?
The author saw MISO start near 3.3 V and then drop to ~1.65 V mid‑session. After adding the buffer, the output wavered up to 3.3 V and mostly recovered, but not 100%. That points to residual loading or timing margin. [Elektroda, Anonymous, post #21683425]
Does a pull‑up on MISO help?
A strong pull‑up can distort the waveform by stretching edges. The author tried one and observed the entire short square wave pulled up. Prefer a buffer with defined thresholds rather than brute pull‑ups on high‑speed lines. [Elektroda, Anonymous, post #21683427]
How do I verify the analyzer vs scope disagreement?
Logic analyzers can decode marginal levels due to hysteresis; scopes reveal analog quality. Trust the scope for amplitude and timing. In the thread, the analyzer decoded fine while the scope showed ragged edges. Fix the analog first. [Elektroda, Anonymous, post #21683411]
What’s a good confirmation that fixes worked?
Repeat WHO_AM_I reads until stable at 0x48 across resets and temperature. Then raise SPI speed incrementally while watching setup/hold on the scope at the PIC pin. Document the highest error‑free rate. [Elektroda, Anonymous, post #21683414]
Any expert tip to remember during debug?
“Look for signals with funny levels.” Measure at the PIC input, not just at the sensor. The right probe point often reveals the real culprit. [Elektroda, Anonymous, post #21683409]
