45.6.2.6.2 Secure Decoder Detection

Position decoders are generally implemented using external photo-detectors, detecting a transmitted or reflected beam. Error detections or no detections may happen, and are due to:

A transmitted beam locally stopped by dust
A reflected beam lost by a mat dust
An additional parasitic reflected beam, due to a gloss metallic dust

When secure detection is enabled (CTRLA.CONF = X4S or CTRLA.CONF = X2S), the Index must be enabled. The Angular counter is restarted on the detection of each overflow. When the Angular counter value reaches its maximum period value (TOP), an Index is expected to be detected. In the same way, if the Index is detected, the Angular counter value is expected to be TOP. If one of these conditions is not met, an error is generated.

Note: The first error generation is masked, as the initial position of the wheel is in unknown state. The first index is used to synchronize the Angular counter on Revolution counter increment.

Figure 45-6. Secure Decoder Detections