what is the meaning hysteresis voltage of schmitt trigger in FPGA ?

[zhangljz]

Hi, I am going to receive many(about 100) input signals with slow rise and fall with FPGA. The frequency may be at 200MHz. I am thinking to reshape the waveform with schmitt triggers of input pins to avoid metastability.

As Cyclone IV handbook said, the minimum hysteresis voltage is 200mv if the IO power supply is 3.3V, in this case what is the Vth+ and Vth- ??

Also I don't know whether the hysteresis voltage of schmitt triggers is programmable and how to do it ?

Thank you.

 

[FvM]

Metastability is caused by violating setup- and hold times, in so far a schmitt trigger can't avoid it.

Hyteresis is the difference between Vth+ and vth-. It may be programmable, but isn't with Cyclone FPGA.

 

[zhangljz]

Hi, FvM, thanks for your correction. For the slow rising and falling input signals, the setup timing will be critical. So if there is a schmitt trigger to make the conversion faster, the setup timing will be better. Am I right?

 

[ads-ee]

A schmitt trigger input isn't meant to speed up conversions. It's meant to improve immunity to non-monotonic rising edges so they don't result in multiple transitions on their output. The hysteresis (larger is better) prevents "dips" in the rising edge from being seen as a low unless it dips below the lower threshold Vth-. If anything schmitt triggers have more delay from input to output, therefore reducing your setup time.

Regards