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Removing GPO Glitches During Reboot of Speedway Revolution Reader

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This article describes how to remove the uncontrolled GPO signals that are generated by the Speedway® Revolution reader when it is rebooted.

GPO Behavior

When the Speedway Revolution reader is rebooted, the GPO signals automatically go into the high state until the boot sequence completes, at which point the GPO signals will drop to the low state. This may be an issue with your application if your system cannot tolerate this uncontrolled behavior (e.g., if a buzzer is connected, it will buzz until these GPO signals are de-asserted).

Solution

To remove this effect, connect an XOR logic gate to two of the GPO pins and, after the boot sequence is complete, use differential signaling to control the OUT signal once the reader is up and running. Here is how to do differential signaling with the GPO signals to control the OUT signal:

1.       Set these two GPO signals to be the complement of the other to generate a high output (e.g., if GPO 1 = 1 and GPO 2 = 0, then OUT = 1)
2.       Set these two GPO signals to be the same value as the other to generate a low output


The circuit connections and the timing diagrams during the boot sequence are show below:


GPO_Glitch.jpg

 

Note that because the timing of the GPO signals is not precisely aligned, as seen in the waveform diagram, small OUT pulses will still be observed during the boot sequence. These are typically too small to affect the system operation and can normally be disregarded.

Hardware Selection

The XOR logic gate selection is typically driven by the output levels you wish to generate. Please visit an electronic parts distributor (e.g., DigiKey) to find the appropriate circuitry.


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