Newbie: Falling edge, what is the threshold? (Xilinx XC9572XL)

Generally the point will lie between 0.8V(Vil) and 2.0V(Vih) which are the guaranteed limits. The XC9572XL has hysteresis on the I/O so level will be different on the way down to the on way up. Threshold will vary with temperature, voltage (and voltage noise) and with silicon batch so don't rely on anything other than guarantee limits.

John Adair Enterpoint Ltd. - Pushing The PicoBlaze.

Those 0.8V is the level, where it is guaranteed to detect a low-level...

So every real device will switch somewhere in between "max. low" and "min. high"

experience "normally" tells me (device independant): - take the supply voltage - divide it by 2 - subtract a small portion 2 to 5%

=> this will be a reasonable switching level (hysteresis NOT taken into account) but you defentively can't rely on it !!

The only thing you can rely on are those mentioned min/max values and using a 'fast' transition (whatever 'fast' means)

Jochen

Jochen

Many thanks John.

- James

Thank you Jochen. I appreciate you passing on your experience too, as someone with very little in this area! ;)

- James

If you want to measure the input threshold, just implement an inverting data path from an input to an output, short the two pins together, and decouple them to ground with a capacitor. You now have an oscillator that switches around the threshold. Peter Alfke, Xilinx

Thanks Peter, that's a great suggestion!

- James

Digital logic IO typically have a SINGLE threshold point despite the guaranteed values of Vil & Vih -- there is only one threshold point -- for TTL logic this has been around 1.5Vdc -- there are some logic which incorporates two threshold points for such as Schmitt Triggers called hysteresis ....

edge.

edge

Thank you Joe. Would the supply being 3.3V (rather than 5V) have any bearing on the 1.5V value?

- James

Not much - it's still around 1.5Vdc ...