Three bugs prevented mpu6000_is_calibrated() from returning true,
blocking arming and balance mode:
1. WHO_AM_I single-attempt: one SPI glitch returning 0x00 caused
icm42688_init() to return -128, skipping mpu6000_calibrate()
entirely. Fix: retry WHO_AM_I up to 3 times with 10ms gaps.
2. icm42688_read() rx[15] uninitialized: if HAL_SPI_TransmitReceive()
failed, garbage stack data was accumulated as gyro bias. Fix: zero-
init rx[15] so failed transfers produce zero data.
3. mpu6000_calibrate() raw uninitialized: UB if icm42688_read() is
a no-op (imu_type mismatch). Fix: zero-init raw each iteration.
Also add SCB_InvalidateDCache_by_Addr() on SPI rx buffers in rreg()
and icm42688_read() for DCache coherency. Currently a no-op (DCache
is not enabled), but required if SCB_EnableDCache() is added — stack
buffers in SRAM2 are in the cacheable memory region on STM32F7.
Fix misleading DCache comment in icm42688.c (claimed DCache was
disabled by main.c; actually SCB_EnableDCache() is never called).
Build: 59904 bytes Flash (+512), 17100 bytes RAM — SUCCESS
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Bug 1 (PRIMARY — DCache/USB coherency):
SCB_DisableDCache() was buried inside icm42688_init(), called ~3.5s
after USB starts. STM32F7 DCache/USB coherency issue: when DCache is
on (enabled by SystemInit()), CPU writes to TX buffers stay in cache
and the USB hardware reads stale SRAM data. Moved SCB_DisableDCache()
to main() before HAL_Init(), ensuring coherency for all USB transfers.
Bug 2 (USB TX corruption):
CDC_Transmit() passed the caller's stack-allocated buf pointer directly
to the USB stack. The USB TXFE interrupt fires asynchronously; by then
the stack buffer may have been modified by the next loop iteration.
CDC_Transmit() now copies into the static UserTxBuffer before handing
off to the USB hardware, ensuring the buffer is stable for the transfer.
Bug 3 (IMU type mismatch → wrong data to balance):
main.c called icm42688_init()/icm42688_read() directly, passing
icm42688_data_t* (raw int16 ax/ay/az/gx/gy/gz) to balance_update()
which expects IMUData* (float pitch/pitch_rate from complementary
filter). Type mismatch produced garbage balance values. Fixed by using
mpu6000_init()/mpu6000_read() which wraps icm42688 + sensor fusion.
Telemetry updated to report fused pitch/rate instead of raw ADC counts.
Also fix icm42688_init() returning 0 on who==0 (no SPI response),
which falsely indicated IMU success.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
