schmitt trigger threshold of 5438

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Posted by david sun77114

on Aug 08 2012 17:47 PM

Hello,

at page 42 of 5438 datasheet, listed schmitt-trigger input threshold when Vcc at 1.8V and 3.0V. if I have a design use 2.0V Vcc. how to calculate the threshold. should I use 1.8V number or 3.0V. is there an equation to estimate the threshold.

Best Regards,

David Sun

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Posted by Jens-Michael Gross

on Aug 09 2012 10:38 AM

Verified Answer

Verified by david sun77114

Guru140205 points

Schmitt-Triggers do not have 'the threshold'. They have two, one for going form low to high, and a lower one for going from high to low. That's why they are schmitt triggers.
The minimum an dmaximum distance between the two is in the datasheet (hysteresis).

MSP is CMOS technology. So besides an offset, voltages scale more or less linearly with VCC. In some datasheets,the values are already given as factors relative to VCC. You can approximate the min/max values pretty good by a linear interpolation between 1.8V and 3V VCC table entries.

Note that the minimum high-going threshold entry is below the maximum low-going threshold. So given teh minimum hysteresis, the one cannot be at minimum when the other is at amximum. Buit on different MSPs, one or the other may be true. So to calculate the worst-case safe area, you'll have to play with the values and eliminate all voltages that are in an either/or region and ensure that your external signals are outside.
I think TI has decided to not put a four-dimensional diagram into the datasheet for obvious reasons :)

david sun77114
is there an equation to estimate the threshold

It's n equations with m variables, resulting in a range of possible values. And, as I said, there are two different thresholds.

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Posted by david sun77114

on Aug 09 2012 11:47 AM

Intellectual795 points

got it, thanks

Posted by david sun77114

on Aug 09 2012 13:47 PM

Intellectual795 points

Jens-Michael

I have one more simple question for 3V environment, to make MSP5438 work safely, Schmitt-Trigger input should be over 2.1V for positive going and lower than 0.75V for negative going. right?

thanks

David

Posted by Jens-Michael Gross

on Aug 10 2012 05:32 AM

Guru140205 points

david sun77114
Schmitt-Trigger input should be over 2.1V for positive going and lower than 0.75V for negative going. right?

Yes. Those are the values that will be accepted for low and high on any MSP for sure. The actual triggering will happen somewhere between them, and vary across different MSPs. But since it is a schmitt-trigger, you will be sure that noise below the hysteresis won't cause multiple triggers even if the input signal transistions are a noisy ramp rather than a clean edge.