Keeping pin from floating

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1

Hey guys. I’m using the attached circuit to turn on/off power to another load. The wire labeled ‘TO MCU’ goes to a digital pin which I control in the code. The issue is when the MCU power cycles, all pins are tri-stated and sometimes it turns power on to the load inadvertently. Is it safe to add a 10K pull-down on that pin to insure it’s always low when the MCU power cycles?

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yup.

You should be fine with 100k

3

markaren1:
You should be fine with 100k

100K? I thought 10K would've been enough. What's the reasoning for the larger resistor, just so I understand it.
4

Less leakage when the microcontroller output needs to charge the gate-capacitance. So quicker on-time.

5

kirash4:

markaren1:
You should be fine with 100k

100K? I thought 10K would've been enough. What's the reasoning for the larger resistor, just so I understand it.

For the same supplies, a 100k resistor will draw 10% of the current of a 10k resistor.

At power up, you have the CPU pin configured as an input connected to a gate. Input leakage is likely to be less than 1uA for the CPU. 1uA through 100k is 0.1V – well below the ON threshold of the FET.

Run these numbers again for 10k and 1uA through 10k is 0.01V. But you are starting to draw current when you apply a logic 1 (3v3 and 10k = 330uA), which is unnecessary.

There is no significant difference between 10k and 100k in your case because the dominant current for the system lies elsewhere. This is more important for low power applications.

-mark

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Thanks for the explanation everyone. Mark, seeing as this will be driven from a single battery and the goal to have it last as long as possible, the less leakage I have, the better.

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That is what is called a stiff/strong pull-up (or pull-down), versus a weak pull-up (or pull-down). The uC usually have weak pull-ups on most of its I/Os (for the PIC that usually controlled by the WPU register). For power conservation you want the weaker pull-up you can afford. That has to be balanced against what you are pulling up, and how hard (or how fast) you need that pull-up to work. In your case it looks like 10k and 100k makes no difference.