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I am having this microcontroller - S32K142 64 pins - 5V Core voltage

I have the below RESET signal connection which is provided by the MCU to the below transistor (which provides RESET to another microcontroller)

enter image description here

Transistor Datasheet BCR116W

I am checking the Vol and Voh compatibility between the microcontroller pin out and the Q0101 Transistor.

Suppose, if the Microcontroller outputs high, from the Microcontroller datasheet, Voh and Vol are not provided directly. They have only mentioned as Vdd-0.8V for Voh and 0.8V for Vol.

My questions :

  1. I am assuming the Voh of the microcontroller to be 3.2V (4V-0.8V=3.2V) (min). Is my assumption correct?

  2. If suppose my microcontroller provides a high to the RESET line, I am guaranteed atleast a 3.2V on the RESET, which happens to be greater than the worst case maximum turn on voltage of the transistor (1.4V). Since, 3.2V > 1.4V - Transistor will turn ON. (Collector current is in the 250uA range) But if I give low at the RESET pin of the microcontroller, since the Vol(max) as specified in the datasheet is only 0.8V, and the maximum turn off voltage of the transistor as given in the datasheet is 0.8V, there seems to be a mismatch in the compatibility, right?

Since, in worst case conditions, my Microcontroller outputs a 0.8V on the RESET for LOW signal, the maximum voltage that needs to be provided for a guaranteed turn off of the transistor at the worst case scenario is less than 0.4V (as mentioned in the table on page 3). Am I correct?

If my microcontroller provides 0.8V and the transistor turn off voltage threshold is 0.4V, the transistor might turn on and trigger falsely right?

Is my understanding correct?

If this is the case, what is the work around in the design?

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1 Answer 1

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The \$V_{oh}\$ and \$V_{ol}\$ specs you're referring to on page 31 of the datasheet aren't really specs on what output voltage the chip will provide. They're test conditions for the \$I_{oh}\$ and \$I_{ol}\$ specs.

The spec is saying, for example, if you externally force the output pin to +0.8 V when the chip is outputting low, the chip is guaranteed to sink at least 5 mA.

Since you don't have to sink any current at all to keep your transistor base below threshold, the outputs in your circuit will certainly achieve a lower output voltage than 0.8 V. In fact, since in your circuit the micro doesn't have to sink any current at all to pull the transistor base low (\$R_B\$ takes care of that), you should achieve very close to 0 V, and you should have no trouble turning the transistor fully off.

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  • \$\begingroup\$ So, I don't have the control on what the output voltage Vol and Voh from the Microcontroller? \$\endgroup\$
    – user220456
    Jun 20, 2020 at 4:54
  • \$\begingroup\$ The Voh and Vol specifications are not provided for this Microcontroller? Or where to find them? \$\endgroup\$
    – user220456
    Jun 20, 2020 at 4:54
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    \$\begingroup\$ @Newbie, what you found is the closest you're likely to get. Maybe there's an app note somewhere that gives a typical I-V curve for the outputs in the high and low states. If you're a big enough customer you could ask your NXP salesperson to provide it. \$\endgroup\$
    – The Photon
    Jun 20, 2020 at 4:57
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    \$\begingroup\$ @Newbie, if instead of pulling the pin low, you just put it in high-Z state (not sinking or sourcing any current), the transistor base would be pulled low by the \$R_B\$ shown in your diagram. So the micro doesn't have to sink any current to ensure the transistor base goes low. \$\endgroup\$
    – The Photon
    Jun 20, 2020 at 5:10
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    \$\begingroup\$ On your other question, if you have a curve tracer (or can make one) you could measure your own I-V curves for these pins. \$\endgroup\$
    – The Photon
    Jun 20, 2020 at 5:11

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