Rabbit 6000 User Manual page 334

The Quadrature Decoders are clocked by the output of Timer A10, which must be fast enough to sample
the inputs properly. Both the I and Q inputs go through a digital filter that rejects pulses shorter than two
clock periods wide so that highest detectable input frequency is one-fourth of the frequency set by Timer
A10. In addition, the clock rate must be high enough that transitions on the I and Q inputs are sampled in
different clock cycles. Input capture may be used to measure the pulse width on the I inputs because they
come from the odd-numbered port bits. The operation of the digital filter is shown below.
The Quadrature Decoder generates an interrupt when the counter increments from 0xFF (0x3FF in 10-bit
mode) to 0x00, or when the counter decrements from 0x00 to 0xFF (0x3FF in 10-bit mode). The timing
for the interrupt is shown below. Note that the status bits in the QDCSR are set coincident with the inter-
rupt, and the interrupt and status bits are cleared by reading the QDCSR.
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Rabbit 6000 User's Manual 334