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Altera_Forum's avatar
Altera_Forum
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15 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 t...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
15 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