Rabbit Rabbit 4000, User Manual

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232 Rabbit 4000 Microprocessor User’s Manual
The Quadrature Decoders are clocked by the output of Timer A10, giving a maximum
clock rate from perclk/2 down to perclk/512. The time constant of Timer A10 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 period wide. 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 interrupt, and the interrupt and status bits
are cleared by reading the QDCSR.
PERI CLOCK
TIMER A10
REJECTED
ACCEPTED
PERI CLOCK
TIMER A10
I or Q INPUT
COUNTER
INTERRUPT
0xFF or 0x3FF
0x00