HCTL-2032 Datasheet, PDF (14/20 Pages) Agilent(Hewlett-Packard) – Quadrature Decoder/Counter Interface ICs

CHA CHB STATE
1
0
1
1
1
2
0
1
3
0
0
4
Figure 13. 4x Decoder Mode
4X Decoder
(Count Up & Count Down)
Pulse
Pulse
Pulse
Pulse
CHA
CHB
STATE
2x
Count Up
1
0
1
Pulse
1
1
2
-
0
1
3
Pulse
0
0
4
-
Figure 14. 2x and 1x Decoder Modes
2x
Count Down
-
Pulse
-
Pulse
1x
Count Up
Pulse
-
-
-
1x
Count Down
-
Pulse
-
-
Design Considerations
The designer should be aware
that the operation of the digital
filter places a timing constraint
on the relationship between
incoming quadrature signals and
the external clock. Figure 12
shows the timing waveform with
an incremental encoder input.
Since an input has to be stable
for three rising clock edges, the
encoder pulse width (t_E - low
or high) has to be greater than
three clock periods (3t_CLK). This
guarantees that the
asynchronous input will be
stable during three consecutive
rising clock edges. A realistic
design also has to take into
account finite rise time of the
waveforms, asymmetry of the
waveforms, and noise. In the
presence of large amounts of
noise, t_E should be much
greater than 3t_CLK to allow for
the interruption of the
consecutive level sampling by
the three- bit delay filter. It
should be noted that a change
on the inputs that is qualified
by the filter will internally
propagate in a maximum of
seven clock periods.
The quadrature decoder
circuitry imposes a second
timing constraint between the
external clock and the input
signals. There must be at least
one clock period between
consecutive quadrature states.
As shown in Figure 13, a
quadrature state is defined by
consecutive edges on both
channels. Therefore, t_ES
(encoder state period) > t_CLK.
The designer must account for
deviations from the nominal 90
degree phasing of input signals
to guarantee that t_ES > t_CLK.
Position Counter
This section consists of a 32- bit
(HCTL- 20XX- XX) binary up/
down counter which counts on
rising clock edges as explained
in the Quadrature Decoder
Section. All 32 bits of data are
passed to the position data
latch. The system can use this
count data in several ways:
A. System total range is 32 bits,
so the count represents
"absolute" position.
B. The system is cyclic with 32
bits of count per cycle. RST/ is
used to reset the counter every
cycle and the system uses the
data to interpolate within the
cycle.
C. System count is >8, 16, 24, or
32 bits, so the count data is
used as a relative or
incremental position input for a
system software computation of
absolute position. In this case
counter rollover occurs. In order
to prevent loss of position
information, the processor must
read the outputs of the IC
before the count increments
one- half of the maximum count
capability. Two's- complement
arithmetic is normally used to
compute position from these
periodic position updates.
D. The system count is >32 bits
so the HCTL- 2032 / 2032- SC
can be cascaded with other
standard counter ICs to give
absolute position.
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