Ghost on the Bus

A colleague wrote firmware to read the WHO_AM_I register from an SPI accelerometer on SPI1. The SPI configuration and byte sequence are correct, but the sensor never responds. Reads come back as 0xFF.

On a scope, the SPI clock and data lines look fine. What's missing?

The sensor's chip-select (CS) is wired to PA4 (active-low).

Find the bug in the starter code and fix it so the WHO_AM_I read returns the sensor's real ID instead of a garbled all-ones byte.