Reset pin connections on STM32F042K6T6 micrcontroller?

[zenerbjt]

Hi,
Page 78 of the STM32F042K6T6 micro appears to have a 100n cap on the reset pin.
Is this essential?

Are there other necessary "auxiliary" components that need connecting up to a STM32F042K6T6 micro schematic?

STM32F042K6T6 datasheet

https://www.st.com/resource/en/datasheet/stm32f042k6.pdf

 

[stenzer]

Hi,

this capacitor has two purposes.

(i) A delayed turn on of your MCU as it creats an RC circuit with the internal pull-up resistor. So it ensures a propper operation of the MCU (after powering your whole circuitry) by a delayed turn on while the rest of the circuitry is propperly supplied.

(ii) De-bouncing of the external reset switch, see [1].


[1] https://hackaday.com/2015/12/09/embed-with-elliot-debounce-your-noisy-buttons-part-i/

BR

--- Updated Aug 23, 2020 ---

.... the datasheet also states this capacitor is required to avoid parasitic resets, which may be caused by EMI or by dirt or moisture creating a low impedance path towards ground.

 

[zenerbjt]

Thanks very much,
We are re-doing a PCB for a company, and notice that they have no such capacitor on the reset pin of their STM32's....i believe we shoudl tell them in case they are having noise issues?....from your kind explanation, it appears that this capacitor is essential?

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Also, please may i ask about the BOOT0 pin (pin 31) of the STM32F042K6T6 micro?..(LQFP32 footprint)..as follows....

We are updating a PCB with a STM32F042K6T6 micro on it.

For some reason, the PCB has pin 31 (BOOT0) of the micro connected to GND. This does not seem right. Please could you confirm that this seems wrong?

STM32F042K6T6 microcontroller datasheet
https://www.st.com/resource/en/datasheet/stm32f042k6.pdf

--- Updated Aug 23, 2020 ---

.... the datasheet also states this capacitor is required to avoid parasitic resets, which may be caused by EMI or by dirt or moisture creating a low impedance path towards ground.

Thanks, and thanks for the link......which shows a series resistor on the reset pin aswell, which is needed because the reset pin is used in programming...so i believe that we will not only need a 100n cap on the reset pin, but also a series 1k resistor?

 

[Aussie Susan]

Thanks, and thanks for the link......which shows a series resistor on the reset pin aswell, which is needed because the reset pin is used in programming...so i believe that we will not only need a 100n cap on the reset pin, but also a series 1k resistor?

If you are programming the MCU on the board then yes, you may well need that.
However if the MCU is being programmed before it is put on the PCB then no you don't need the series resistor.
You may need to go back to the client and find out a few more details before you create the new PCB.
Susan

 

[stenzer]

Hi,

to check the existing layout and to get an impression how a recommended and proven schematic looks like have a look at [1]. It's the user manual of the STM32F042 offered by ST and the schematic is shown on page 33.

[1] https://www.st.com/resource/en/user...nucleo32-boards-mb1180-stmicroelectronics.pdf

BR

 

[zenerbjt]

Thanks, i notice the Reset pin has a 100n cap attached on page 33.....but since we will be programming it regularly on the board i dont see how we can have that....since it would be getting shorted and powered continuously.....so i believe we would need the series resistor? Microchip recomends such a resistor.
Also, I remember from working with PICs that if one missed out the cap on the reset pin ("MCLR"), then the PIC was horrendously affected by noise.

 

[stenzer]

Hi,

sorry, I don't get your point. What are you concerned of? Maybe you can upload a schematic of your intended reset programming design.

The reset pin is also used by the programmer to reset the MCU after flashing. So you may introduce an additional low valued series resistor between the reset pin and the pin where your programmer is attached to "softly" discharge the capacitor when the programmer pulls the MCU reset pin towards GND. I used the STM32 controllers in several designs and I always programmed it on the PCB via SWD or JTAG, I never used an additional series resistor. Also no series resistor is used in the UM1956.

BR

 

[FvM]

For some reason, the PCB has pin 31 (BOOT0) of the micro connected to GND. This does not seem right. Please could you confirm that this seems wrong?

Did you read the reference manual. Pulling down BOOT0 has a purpose, do you understand the application requirements?

As for the 100nF capacitor, it's a rather big value, to avoid capacitively coupled noise triggering the reset, we use 100 pF to 1 nF.