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DWiśn.1
DWiśn.1
Associate II
Jun 17, 2021 11:06 AM UTC
Solved

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

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.

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waclawek.jan
waclawek.janBest answer
Super User
Jun 17, 2021 11:20 AM UTC

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

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6 replies

waclawek.jan
waclawek.janBest answer
Super User
Jun 17, 2021 11:20 AM UTC

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
TDK
Jun 17, 2021 12:42 PM UTC

> 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.

0693W00000BbGGIQA3.png

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DWiśn.1
DWiśn.1Author
Associate II
Jun 21, 2021 9:23 AM UTC

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.

Uwe Bonnes
Uwe Bonnes
Chief
Jun 17, 2021 12:48 PM UTC

How do such "nominal" values help anybody?

TDK
TDK
Jun 21, 2021 2:24 PM UTC

> 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.

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DWiśn.1
DWiśn.1Author
Associate II
Jun 23, 2021 9:51 AM UTC

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

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