42.6.2.6.5 Missing Pulse Detection and Auto-Correction

The PDEC embeds circuitry to detect and correct errors that may result from contamination on optical disks or other sources producing quadrature phase signals.

The auto-correction works in QDEC X4 mode only. A missing pulse on a phase signal is automatically detected, and the pulse count reported in the Angular part of COUNT is automatically corrected.

There is no autocorrection if both phase signals are affected at the same location on the input signals, because the autocorrection requires a valid phase signal to detect contamination on the other phase signal.

If the quadrature source is undamaged, the number of pulses counted for a predefined period of time must be the same with or without detection and auto-correction. Therefore, if the measurement results differ, a contamination exists on the source producing the quadrature signals. This does not substitute the measurements of the number of pulses between two index pulses (if available) but provides an additional method to detect damaged quadrature sources.

When the source providing quadrature signals is strongly damaged, potentially leading to a number of consecutive missing pulses greater than 1, the quadrature decoder processing may be affected.

The Maximum Consecutive Missing Pulses bits in Control A register (CTRLA.MAXCMP) define the maximum acceptable number of consecutive missing pulses. If the limit is reached, the Missing Pulse Error flag in Status register (STATUS.MPERR) is set. The Error Interrupt flag is set (INTFLAG.ERR) and an optional interrupt can be generated.