The QDEC is equipped with a circuitry which detects and corrects some errors that may result from contamination on optical disks or other materials producing the quadrature phase signals.
The detection and autocorrection only works if the Count mode is configured for both phases (EDGPHA = 1 in TC_BMR) and is enabled (AUTOC = 1 in TC_BMR).
If a pulse is missing on a phase signal, it is automatically detected and the pulse count reported in the CV field of the TC_CV0/1 is automatically corrected.
There is no detection if both phase signals are affected at the same location on the device providing the quadrature signals because the detection requires a valid phase signal to detect the contamination on the other phase signal.
If a quadrature device is undamaged, the number of pulses counted for a predefined period of time must be the same with or without detection and autocorrection feature.
Therefore, if the measurement results differ, a contamination exists on the device producing the quadrature signals.
This does not substitute the measurements of the number of pulses between two index pulses (if available) but provides a complementary method to detect damaged quadrature devices.
When the device providing quadrature signals is severely damaged, potentially leading to a number of consecutive missing pulses greater than 1, the downstream processing may be affected. It is possible to define the maximum admissible number of consecutive missing pulses before issuing a Missing Pulse Error flag (MPE in TC_QISR). The threshold triggering an MPE flag report can be configured in TC_BMR.MAXCMP. If the field MAXCMP is cleared, MPE never rises. The flag MAXCMP can trigger an interrupt while the QDEC is operating, thus providing a real time report of a potential problem on the quadrature device.