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Altera_Forum's avatar
Altera_Forum
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16 years ago

External trigger so easy?

I just want to use the signal generator to trigger one IO pin of FPGA externally. On FPGA , this trigger sigal will be captured on the rising edge.

But, when I just connect the IO pin on FPGA to the cable which will be connected to the sigal generator or oscillograph, the trigger starts, it works so easy that I can't control it, what's interesting is that it works even when I toutch the pin.

Then I measure the voltage of my fingers which is about 500 mV.

FPGA Information:

LVTTL selected, 10 pf loaded, dual-in-line pin used as the input trigger.

Does anybody kind of use external trigger, and know why this happened?

John

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  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    16 years ago

    If your input floats, 500mV is enough to get it triggered. You need at least a pull-up to prevent this. I don't know if the internal pull-up is sufficient for this.

    More than sufficient is the following circuit: pull-up resistor of 10K from pin to power, a Capacitor of 100pF from pin to ground, a series resistor of 100 Ohm from your board input to pin and a pair of protection diodes from board input to power and ground (BAV99). You can change the component properties depending on your signal speed.

    The circuit not only prevents triggering, but also protects your FPGA from damaging.

    Good luck, Ton