[STM32F373] I/O characteristics - Schmitt trigger hysteresis details

[DWiśn.1]

Hi!

I measure the frequency signal on the input of STM32F373. General Purpose Timer in Input Capture configuration is being used to do so. But when the rising edge width of the input signal is around 110 ms, I detect some extra, unexpected pulses. I suspect signal voltage to be in the Undefined range.

STM32F373xx datasheet on page 84 states that Schmitt trigger hysteresis has 100 mV for FT and FTf I/O.

I could find information neither in documentation nor on the Internet about the details of the hysteresis. Is it outside the range, inside or maybe symmetrically distributed around VIL, VIH values?

Thank you in advance for any information.

[waclawek.jan · Accepted solution]

Probably inside, otherwise VIL/VIH specification would not make sense.

For better control of threshold and/or hysteresis, you may want to use a comparator (or maybe even an opamp).

JW

[TDK]

> I could find information neither in documentation nor on the Internet about the details of the hysteresis.

It's in the datasheet. Nominal is 100mV. No min/max is specified, but the limits on logic levels would imply a max.

[Uwe Bonnes]

How do such "nominal" values help anybody?

[DWiśn.1]

As I wrote above, I found information about the nominal value. But nowhere was the direction of the hysteresis specified, and this interested me.

Thank you for your reply.

[TDK]

> As I wrote above, I found information about the nominal value. But nowhere was the direction of the hysteresis specified, and this interested me.

I missed that part of the post.

Hysteresis is applicable in both directions and it is entirely within the "undefined" range below logic low and high. Those are the only guarantees you get.

Due to part to part variation and the desire of the manufacturer to meet the guarantees it's putting out, it's usually in the middle of the range.

[DWiśn.1]

This clears things out. Thank you all for your answers.