Xilinx MIcroBlaze Development Spartan-3E 1600E Kit User Manual page 18

Chapter 2: Switches, Buttons, and Knob
Figure 2-6: Push-Button Switches Require Internal Pull-up Resistor in FPGA Input
Rotary Shaft Encoder
In principal, the rotary shaft encoder behaves much like a cam, connected to central shaft.
Rotating the shaft then operates two push-button switches, as shown in
Depending on which way the shaft is rotated, one of the switches opens before the other.
Likewise, as the rotation continues, one switch closes before the other. However, when the
shaft is stationary, also called the detent position, both switches are closed.
Closing a switch connects it to ground, generating a logic Low. When the switch is open, a
pull-up resistor within the FPGA pin pulls the signal to a logic High. The UCF constraints
in
The FPGA circuitry to decode the 'A' and 'B' inputs is simple, but must consider the
mechanical switching noise on the inputs, also called chatter. As shown in
chatter can falsely indicate extra rotation events or even indicate rotations in the opposite
direction! See the Rotary Encoder Interface reference design
example.
16
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Rotary / Push Button
3.3V
A pull-up resistor in each input pin
generates a '1' for an open switch.
See the UCF file for details on
specifying the pull-up resistor.
Rota ry Shaft
Encoder
GND
Figure 2-7: Basic example of rotary shaft encoder circuitry
Figure 2-9 describe how to define the pull-up resistor.
MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
FPGA I/O Pin
ROT_CENTER Signal
Pin
A='0'
B='1'
in"Related Resources"
UG257 (v1.1) December 5, 2007
R
UG257_02_06_060906
Figure 2-7.
FPGA
Vcco
Vcco
UG257_02_07_060206
Figure 2-8, the
for an