STM32F103 - No Internal pull-up on the NRST pin

samuel23 · ‎2017-02-23

Posted on February 23, 2017 at 11:48

Hi,

In the current datasheet of STM32F103 (DocID13587 Rev 17) it is written:

5.3.14 NRST pin characteristics

The NRST pin input driver uses CMOS technology.

It is connected to a permanent pull-up resistor, RPU (see Table 35).

In few project we did, we noticed there is no internal pull-up on this pin. (and we needed to add an external pull-up to ensure the mcu work).

Thanks!

#pull-up #stm32f103 #nrst-pin

waclawek.jan · ‎2017-02-23

Posted on February 23, 2017 at 12:21

Which part exactly? How do you know the pullup is not there?

JW

samuel23 · ‎2017-02-23

Posted on February 23, 2017 at 13:22

We used the STM32F103RBT6.

Some students measured floatting values on this NRST pins.

This measure confirm the symptom: these MCU did not launch the program until an external pull-up was added.

waclawek.jan · ‎2017-02-23

Posted on February 23, 2017 at 20:07

What was the circuitry to which NRST was connected before adding teh pullup?

JW

samuel23 · ‎2017-02-24

Posted on February 24, 2017 at 08:52

Any circuitry was linked to NRST.

These MCU are soldered on custom board... and for many of them, the NRST pin was not linked to any net.

I have done a test with a working well MCU :

I measure an infinite impedance between NRST and VDD when the MCU is power off --> So I guess the pull-up is not permanent, and may be software linked during the hard boot process of the MCU?

Maybe something hurts the MCU in the students board. (due to decoupling, of miss solder in other pins...or what else..)

waclawek.jan · ‎2017-02-24

Posted on February 24, 2017 at 09:05

I measure an infinite impedance between NRST and VDD when the MCU is power off --> So I guess the pull-up is not permanent, and may be software linked during the hard boot process of the MCU?

Quoting from the datasheet, the NRST pin characteristics chapter:

2. The pull-up is designed with a true resistance in series with a switchable PMOS. This PMOS contribution

to the series resistance must be minimum (~10% order) .

I admit I don't know why is there a switchable transistor and what are the exact circumstances of swiching it on.

I don't use the 'F1xx. With 'F4xx/'F0xx, I have always the 100nF capacitor connected to NRST, as recommended by the DS, and never encountered problems.

Are all VDD/VDDA and VSS/VSSA pins connected properly? Does the power supply's voltage rise monotonically?

JW

samuel23 · ‎2017-02-24

Posted on February 24, 2017 at 09:17

Thank you for these answers.

Unfortunately, I can not go further into the investigation for now, having not the PCBs in hand.

But I keep these ideas...

Anyway, we will add in the instructions the necessity of adding the 100nF on NRST and a footprint for an external pull-up + the link to SWD connector to ensure...

Thanks !