3-26
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Chapter 3
Counters
Figure 3-29.
X4 Encoding
Channel Z Behavior
Some quadrature encoders have a third channel, channel Z, which is also referred to as the index
channel. A high level on channel Z causes the counter to be reloaded with a specified value in a
specified phase of the quadrature cycle. Depending on configuration, this reload can occur in any
one of the four phases in a quadrature cycle.
Channel Z behavior—when it goes high and how long it stays high—differs with quadrature
encoder designs. Refer to the documentation for your quadrature encoder to obtain timing of
channel Z with respect to channels A and B. You must then ensure that channel Z is high during
at least a portion of the phase you specify for reload.
In Figure 3-30, the reload phase is when both channel A and channel B are low. The reload
occurs when this phase is true and channel Z is high. Incrementing and decrementing takes
priority over reloading. Thus, when the channel B goes low to enter the reload phase, the
increment occurs first. The reload occurs within one maximum timebase period after the reload
phase becomes true. After the reload occurs, the counter continues to count as before. The figure
illustrates channel Z reload with X4 decoding.
Figure 3-30.
Channel Z Reload with X4 Decoding
Ch A
Ch B
Co
u
nter V
a
l
u
e
5
6
8
9
10
10
11
11
12
12
1
3
1
3
7
5
6
8
7
9
Ch A
Ch B
Co
u
nter V
a
l
u
e
5
6
A = 0
B = 0
Z = 1
Ch Z
M
a
x Time
bas
e
8
9
0
2
1
7
4
3
