16 MicroBlaze Development Kit Spartan-3E 1600 Edition User Guide
www.xilinx.com UG257 (v1.1) December 5, 2007
Chapter 2: Switches, Buttons, and Knob
R
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 Figure 2-7.
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 Figure 2-9 describe how to define the pull-up resistor.
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 Figure 2-8, the
chatter can falsely indicate extra rotation events or even indicate rotations in the opposite
direction! See the Rotary Encoder Interface reference design in“Related Resources” for an
example.
Figure 2-6: Push-Button Switches Require Internal Pull-up Resistor in FPGA Input
Pin
UG257_02_06_060906
Rotary / Push Button
ROT_CENTER Signal
3.3V
FPGA I/O Pin
Figure 2-7: Basic example of rotary shaft encoder circuitry
GND
Vcco
Vcco
A=‘0’
B=‘1’
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.
FPGA
UG257_02_07_060206
Rotary Shaft
Encoder
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