Revision(s) Affected

0, A, B, C, D, E, F and G

Details

After a DSP reset, the QEP module fails to detect the first transition that occurs on QEP input pins. (This problem also manifests itself when an external clock is used for the EV timers.) Therefore, if the first transition occurs after a GP timer has been initialized and enabled as the QEP counter (that is, to use QEP as source of clock), the first transition will not be counted by the GP timer. The result is an error of one count in the GP timer out of a total of 1024 counts for a 256-line encoder, or 4096 counts for a 1024-line encoder. However, the issue is not a concern under any of the following conditions:

1. The first transition happensbefore the GP timer is initialized and enabled as QEP counter. This ensures that all transitions are counted after initialization.
2. After the first index pulse is received and if the index pulse is used to recalibrate the GP Timer (through capture interrupt). The recalibration corrects the error in the GP timer; therefore, from the time the first index pulse is received, the QEP counter becomes accurate.

Workaround(s)

1. Make the first transition happen before the GP timer is initialized and enabled as QEP counter. This is usually the case because typically the rotor shaft is locked to a known position before the GP timer is initialized. Locking the rotor shaft will generate transitions on QEP input pins, unless the rotor shaft is exactly aligned to the known position (which is a rare case). Disturbing the rotor shaft on purpose takes care of the rare case.
2. Use the index pulse of the encoder to recalibrate the GP timer used as QEP counter.
3. The counter has to be forced to count before the application actually uses the QEP. During initialization, configure the internal clock (HSPCLK) to be the counter source. After the first count is done, the counter should be reconfigured for external signals (QEP/TCLKIN) and reset to 0. Now the counter will also count the first edge of the QEP.