NXP Semiconductors freescale KV4 Series, Reference Manual

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41.5.1 Positive versus Negative Direction
A typical quadrature encoder has three outputs:
1. PHASEA signal
2. PHASEB signal
3. INDEX pulse (not shown)
If PHASEA leads PHASEB, then motion is in the positive direction. If PHASEA trails
PHASEB, then motion is in the negative direction. Transitions on these phases can be
integrated to yield position or differentiated to yield velocity. The quadrature decoder is
designed to perform these functions in hardware.
41.5.2 Prescaler for Slow or Fast Speed Measurement
For a fast moving shaft encoder, the speed can be computed by calculating the change in
the position counter per unit time, or by reading the position difference counter register
(POSD) and calculating speed. For applications with slow motor speeds and low line
count quadrature encoders, the timer module enables high resolution velocity
measurements by measuring the time period between quadrature phases. The timer
module uses a 16-bit free running counter operated from a prescaled version of the IPBus
clock. The prescaler divides the IPBus clock by values ranging from 1 to 128. A 60 MHz
IPBus clock frequency would yield a resolution of from 17 ns to 2.1 μs and a maximum
count period of from 1.09 ms to 140 ms. For example, with a 1000 tooth decoder, speeds
could be calculated down to 0.11 rpm using a prescaler.
41.5.3 Holding Registers and Initializing Registers
Hold registers are associated with three counters:
1. Position
2. Position difference
3. Revolution
Chapter 41 Quadrature Encoder/Decoder (ENC)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
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