Strange behavior on open-drain output mode for blue pill (STM32F103C8T6)

[Luki_Grin]

Guys, I need to put 5V on a pin. For this, I configured my pin as an open-drain output with a pull-up resistor (10k) between the pin and the 5V line. I checked the 5V-tolerant pins in the Blue Pill datasheet and tested many of them, but the conclusion is always the same: the output is 3.7V when the pin is in high impedance. I used Keil uVision to write my code in C (at startup, I only used CMSIS and DEVICE for pre-configuration). My code is below. I used PB10 as a reference.

#include "stm32f10x.h"

int main(){
 RCC->APB2ENR |= (1 << 3); // Clock for GPIOB

 // Open-drain output and HIGH impedance PB10
 GPIOB->CRH &= ~(0b1111 << 8); // Cleaning bits
 GPIOB->CRH |= (0b0111 << 8); // Setting up bits

 GPIOB->ODR |= (1 << 10); // HIGH impedance
};

 

Could you guys please tell me what I did wrong? 

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Edited to apply source code formatting - please see How to insert source code for future reference.

[TDK]

Maybe the overwhelmingly likely counterfeit chip on the board doesn't have 5 V tolerant pins.

[Luki_Grin]

Thanks for your comment! I didn't think it was a clone, as the debugger with STM32CubeIDE worked perfectly fine. However, after checking some IDs, I found very strange values, so it's probably fake =(

[Peter BENSCH]

You have (at least) made two mistakes:

• You are using a so-called Blue Pill that has only been used with counterfeits for years (and you are asking for a solution for the counterfeit in the forum of the original manufacturer).

• You are confusing 5V tolerance (of an input) with 5V capability of an output

Regards
/Peter

[Luki_Grin]

Thanks for the clarification, Peter. I'm new to the world of microcontrollers, and I'm diving into bare metal to understand how things really work. Didn't know there were clones. I will probably move to a NUCLEO one in the future (to avoid spending many hours on these bugs). 

"You are confusing 5V tolerance (of an input) with 5V capability of an output" --> Could you explain what you mean? I know the output voltage on a pin is normally 3.3V, but if I configure the output as open-drain (high impedance) and connect a pull-up resistor to it, I can get higher values of voltage, right? 

[Peter BENSCH]

This depends on:

• whether the specific GPIO has an alternative analogue function
• and, as mentioned, whether it is an original or a counterfeit.

Details on GPIO behaviour can be found for the original device in the data sheet and in the Reference Manual RM0008.

Hope that helps?

Regard
/Peter