UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 548 of 841
NXP Semiconductors
UM10360
Chapter 26: LPC176x/5x Quadrature Encoder Interface (QEI)
number of edges counted in a given time period is directly proportional to the velocity of
the encoder. Setting the reset velocity bit (RESV) has the same effect as an overflow of
the velocity timer, except that the setting the RESV bit will not generate a velocity
interrupt.
The following equation converts the velocity counter value into an RPM value:
RPM = (PCLK * QEICAP * 60) ÷ (QEILOAD * PPR * Edges)
where:
• PCLK is the peripheral clock rate for the QEI block. See Section 4.7.3 for more on the
possibilities for PCLK).
• QEICAP is the captured velocity counter value for the last velocity timer period.
• QEILOAD is the velocity timer reload value.
• PPR is the number of pulses per revolution of the physical encoder used in the
application
• Edges is 2 or 4, based on the capture mode set in the QEICON register (2 for
CapMode set to 0 and 4 for CapMode set to 1)
For example, consider a motor running at 600 RPM. A 2048 pulse per revolution
quadrature encoder is attached to the motor, producing 8192 phase edges per revolution
(PPR * Edges). This results in 81,920 pulses per second (the motor turns 10 times per
second at 600 RPM and there are 8,192 edges per revolution). If the timer were clocked at
10,000 Hz, and the QEILOAD was 2,500 (corresponding to ¼ of a second), it would count
20,480 pulses per update. Using the above equation:
RPM = (10000 * 1 * 20480 * 60) ÷ (2500 * 2048 * 4) = 600 RPM
Now, consider that the motor is sped up to 3000 RPM. This results in 409,600 pulses per
second, or 102,400 every ¼ of a second. Again, the above equation gives:
RPM = (10000 * 1 * 102400 * 60) ÷ (2500 * 2048 * 4) = 3000 RPM
These are simple examples, real-world values will have a higher rate for PCLK, and
probably a larger value for QEILOAD as well.
26.4.4 Velocity compare
In addition to velocity capture, the velocity measurement system includes a
programmable velocity compare register. After every velocity capture event the contents
of the velocity capture register (QEICAP) is compared with the contents of the velocity
compare register (VELCOMP). If the captured velocity is less than the compare value an
interrupt is asserted provided that the velocity compare interrupt enable bit is set. This can
be used to determine if a motor shaft is either stalled or moving too slow.
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