3
\$\begingroup\$

I have a question about the schematic design of the STM32 "Blue Pill" board. In the datasheet it says there is an internal pull-up resistor of ~40k is included in the MCU for the NRST pin connection (datasheet):

enter image description here

enter image description here

But in the schematic design of the STM32 "Blue Pill" there is included a 10k pull-up resistor externally: enter image description here

Wouldn't this make the equivalent parallel resistance to be 10k//40k = 8k and therefore increase the pull-up current and reduce the efficiency of the NRST pin. Making the pull-up resistor smaller would make it a stronger pull-up but in this case I don't see it necessary since the NRST connection would be short and near the MCU anyway. Is there a reason behind this decision that I am unaware of?

\$\endgroup\$
2
  • 2
    \$\begingroup\$ Notice that NRST is brought to a header pin, which means that a user has access. Who knows what a user might tie in? Perhaps the designer decided to strengthen pull-up to mitigate externally-brought-in noise on this line. Not much worse than a randomly-resetting microcontroller. \$\endgroup\$
    – glen_geek
    Jul 9, 2022 at 13:01
  • 2
    \$\begingroup\$ @glen_geek That is exactly the reason a product like that must have a manual. The pin already has a 100nF capacitor for reset timing, so it has fairly low AC impedance to begin with, not very prone to pick up noise. Plus it is an open-drain IO pin, which the user should know too in order to keep the MCU and connected thing operational without damage. \$\endgroup\$
    – Justme
    Jul 9, 2022 at 13:43

1 Answer 1

4
\$\begingroup\$

Yes, nRST has internal pull-up already. It does not need an external pull-up. Adding an external pull-up in addition to the internal will charge up the capacitor faster as per your calculations.

External pull-up is not mentioned or suggested. A long time ago STM32 datasheets mentioned that an external pull-up could be used in an environment with lots of electromagnetic interference. But it has been removed.

Why the pull-up is there is unknown. Some other MCUs don't have internal pull-ups on their reset pins, so it is a reasonable assumption to have a pull-up on reset pin.

That is not the only thing wrong or against datasheet suggestions in the Blue Pill, so take it as an example how not to make a product by ignoring datasheets, hardware getting started guides, and application notes of the MCU manufacturer.

\$\endgroup\$
2
  • \$\begingroup\$ Yes, I figured out many other problems along the way, like some cap values and placements, etc., but I want to ask another question about the datasheet. the first picture (Table 38. NRST pin characteristics) uses the adjective "Weak" about the internal pull-up resistor. What exactly is meant by that? \$\endgroup\$
    – Berk
    Jul 11, 2022 at 11:48
  • \$\begingroup\$ @Berk Everything is relative, but since the internal resistance is about 40 kilo-ohms, it is not very strong in the context. \$\endgroup\$
    – Justme
    Jul 11, 2022 at 11:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.