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# How to calculate STM32H IO driver capacitance??

Associate II

Similar to I2C bus capacitance, I want to calculate individual pin driver and output capacitance.

Is there any related documents for such?

5 REPLIES 5

My copy of stm32h753xi data sheet (DS12117 Rev 7) gives the capacitance in table 59 of section 6.3.15 I/O port characteristics

Cio is typically 5 pF.

But what do you mean by output capacitance?

Do you mean capacitance when the pin is programmed as a push-pull output?

Because under those circumstances, ST give a user-programmable "speed selection" to allow the user to trade-off data rate and EMC emissions, and I imagine that dominates the rise time rather than some effective capacitance - unless you have a very large external load capacitance. And table 62 explores that.

Hope this helps,

Danish

Associate II

Thanks for the info Danish.

But i have a doubt,

In the table 62, they have given rise time fall time ratio for different frequencies under different speed grades along with that they have given capacitance value for different voltage levels.

Wether the capacitance mentioned in the table is the maximum output capacitance offered by the MCU?? or is it something else?

That's the rise-time you'll see if you connect an external capacitor of that value. The internal capacitance is still about 5 pF. (Slew-rate control is not simply by switching in an additional capacitance),

What happens if you connect a load with larger capacitance?

Well then you'll run into the maximum drive current capability of the MOSFETs used to drive each GPIO pin. They will try to drive about 20 mA each, and you might soon hit the absolute maximum rating of 140 mA for all the I/O pins if several change simultaneously. To prevent damage you might need to add external series resistance or an external buffer.

Associate II

"That's the rise-time you'll see if you connect an external capacitor of that value."

This means that for the given external load capacitance (50pF, 30pF, 10pF) i will get the coressponding rise and fall time in my signal?

It is the load on the output pin. Capacitance here being the resistance to change​ presented by the PCB wiring and input(s) it is expected to drive.

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