Decoupling capacitor at reset pin of Microcontroller

[Techman_7]

Hello,

We are working on BLDC motor control project for functional safety level ASIL B (automotive application).

Is it required to have a decoupling capacitor at the reset pin?

I think its recommended to have a capacitor at reset pin, the logic is to prevent noise at reset pin & also to have power on reset.

Microcontroller is AURIX TC364 in our application.

Can anybody let me know?

 

[barry]

a cap will help mitigate noise, but won’t do anything for power on reset unless there’s also a resistor. but if you’re worried about noise on your reset pin, what about your address and data pins?

 

[Techman_7]

Obviously there is resistor in series with the capacitor.

 

[FvM]

Obviously there is resistor in series with the capacitor.

Why particularly? I would expect a parallel resistor in case of a simple RC POR circuit. TC364 datasheet seems to expect PORST pin driven by an external power supervisory circuit, this may be relevant for functional safety.

 

[KlausST]

Hi,

I don´t see why it should be obvious, either.
If there is an R, then it combines to an RC low pass filter.

A capacitor may reduce the symptom, but won´t cure the cause. The "cause" may distrub other signals as well, thus curing the cuase is the preferred way.

Klaus

 

[Techman_7]

Hi,

I don´t see why it should be obvious, either.
If there is an R, then it combines to an RC low pass filter.

A capacitor may reduce the symptom, but won´t cure the cause. The "cause" may distrub other signals as well, thus curing the cuase is the preferred way.

Klaus

Can you pls elaborate "cause" sentence.

--- Updated Mar 17, 2024 ---

Why particularly? I would expect a parallel resistor in case of a simple RC POR circuit. TC364 datasheet seems to expect PORST pin driven by an external power supervisory circuit, this may be relevant for functional safety.

Actually, it would be 1 R in series & 2nd R in parallel to cap. Additional 3rd R can be connected in series with RESET pin.

This will for perfect POR circuit.

The question is to have C or not?

 

[KlausST]

Hi,

Can you pls elaborate "cause" sentence.

You wrote:

to prevent noise

There will be a reason for the " noise".
Remove te reason --> remove the noise --> no problem on the RESET pin, no problem on other signals.

The question is to have C or not?

There is no general answer. If there was, then you will find it recommended in the datasheet.

Depends on: PCB layout, noise sources, noise level, timing, cost, ...and a lot more...

Klaus

 

[D.A.(Tony)Stewart]

https://www.infineon.com/dgdl/Infineon-AURIX_TC36x_AA_Step-ErrataSheet-v02_02-EN.pdf?fileId=5546d4627506bb320175229e9ae768b3

 

[barry]

https://www.infineon.com/dgdl/Infineon-AURIX_TC36x_AA_Step-ErrataSheet-v02_02-EN.pdf?fileId=5546d4627506bb320175229e9ae768b3

View attachment 189480

Well, that has nothing to do with anything.

 

[Techman_7]

https://www.infineon.com/dgdl/Infineon-AURIX_TC36x_AA_Step-ErrataSheet-v02_02-EN.pdf?fileId=5546d4627506bb320175229e9ae768b3

View attachment 189480

The solution / suggestion to query not reflecting here.

 

[Aussie Susan]

I know this is a different MCU (it's from the dsPIC33EP512MU810 datasheet) but this is typical of what I've seen elsewhere.
The idea is to hold the \MCLR\ pin low while the capacitor is charging during power-on to allow the power supply to stabilise.
It is also noted (elsewhere) that this arrangement is 'optional' and simply holding the reset pin high via a resistor to Vdd is quite OK.
I suspect that the OP is dealing with a case where the power supply will have a lot of spikes due to the motor(s) and automotive applications are well known for having very noisy power. If the Vdd can drop below the point where the reset is triggered then having an arrangement such as shown above may well help.
Personally I'd rather protect Vdd as a whole against noise rather than the reset pin only.
Susan