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231
22. Q
UADRATURE
D
ECODER
22.1 Overview
The Rabbit 4000 has a two-channel Quadrature Decoder that accepts inputs via specific
pins on Parallel Ports D and E. Each channel has two inputs, the in-phase (I) input and the
90 degree or quadrature-phase (Q) input. An 8 or 10-bit up/down counter counts encoder
steps in the forward and backward directions, and provides interrupts when the count goes
from 0x00 to 0xFF or from 0xFF to 0x00. An interrupt can occur each time the count over-
flows or underflows. The Quadrature Decoder contains digital filters on the inputs to pre-
vent false counts. The external signals are synchronized with an internal clock provided by
the output of Timer A10.
Each Quadrature Decoder channel accepts inputs from either the upper nibble or lower
nibble of Parallel Ports D and E. The I signal is input on an odd-numbered port bit, while
the Q signal is input on an even-numbered port bit. There is also a disable selection, which
is guaranteed not to generate a count increment or decrement on either entering or exiting
the disabled state.
The operation of the counter as a function of the I and Q inputs is shown below.
00
01 02 03 04 05 06 07 08 07 06 05 04 03 02 01 00 FF
I
INPUT
Q
INPUT
8-BIT
COUNTER
Interrupt
000 001 002 003 004 005 006 007 008 007 006 005 004 003 002 001 0003FF
10-BIT
COUNTER
FF
3FF
Interrupt
Содержание Rabbit 4000
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