SPRUHM9H October 2014 – May 2024 TMS320F28075 , TMS320F28075-Q1 , TMS320F28076
The eQEP peripheral includes an integrated edge capture unit to measure the elapsed time between the unit position events as shown in Figure 16-15. This feature is typically used for low-speed measurement using the following formula:
where:
The eQEP capture timer (QCTMR) runs from prescaled SYSCLKOUT and the prescaler is programmed by the QCAPCTL[CCPS] bits. The capture timer (QCTMR) value is latched into the capture period register (QCPRD) on every unit position event and then the capture timer is reset, a flag is set in QEPSTS:UPEVNT to indicate that new value is latched into the QCPRD register. Software can check this status flag before reading the period register for low speed measurement, and clear the flag by writing 1.
Time measurement (ΔT) between unit position events is correct if the following conditions are met:
If the QEP capture timer overflows between unit position events, then the timer sets the QEP capture overflow flag (QEPSTS[COEF]) in the status register and the QCPRDLAT register is set to 0xFFFF. If direction change occurs between the unit position events, then the error flag is set in the status register (QEPSTS[CDEF]) and the QCPRDLAT register is set to 0xFFFF.
The Capture Timer (QCTMR) and Capture Period register (QCPRD) can be configured to latch on the following events:
If the QEPCTL[QCLM] bit is cleared, then the capture timer and capture period values are latched into the QCTMRLAT and QCPRDLAT registers, respectively, when the CPU reads the position counter (QPOSCNT).
If the QEPCTL[QCLM] bit is set, then the position counter, capture timer, and capture period values are latched into the QPOSLAT, QCTMRLAT and QCPRDLAT registers, respectively, on unit time out.
Figure 16-17 shows the capture unit operation along with the position counter.
Velocity calculation equation:
where:
Unit time (T) and unit period (X) are configured using the QUPRD and QCAPCTL[UPPS] registers. Incremental position output and incremental time output is available in the QPOSLAT and QCPRDLAT registers.
Parameter | Relevant Register to Configure or Read the Information |
---|---|
T | Unit Period Register (QUPRD) |
ΔX | Incremental Position = QPOSLAT(k) - QPOSLAT(K-1) |
X | Fixed-unit position defined by sensor resolution and QCAPCTL[UPPS] bits |
ΔT | Capture Period Latch (QCPRDLAT) |