If i am using a button conntected to pin 1 of GPIOA ..
GPIOA_ENABLE_CLOCK();
GPIOA->MODER |=PIN_INPUT;
GPIOA->PUPD |=NO_PULLUP_NO_PULL_DOWN;
GPIOA->ODR |=(1 << 1); //this for external pull up resistor
when i debug GPIOA IDR I found that IDR = 0x0000DFD9 .
which means that pin 1 level is low , although i am connecting it to an external pull resistor ... why GPIOA->IDR is not 0x0000DFDB???why it is set to 0 pin 1?????

 

The Discovery board manual (UM1670, rev 3) shows PA1 connected to INT1 of an L3GD20. From its data sheet (DocID023469 Rev 2), the description of CTRL3, appears to default the INT1 pin to push-pull/active high, which (to me) means that at startup it drives PA1 low. That probably beats your pullup.

 

The Discovery board manual (UM1670, rev 3) shows PA1 connected to INT1 of an L3GD20. From its data sheet (DocID023469 Rev 2), the description of CTRL3, appears to default the INT1 pin to push-pull/active high, which (to me) means that at startup it drives PA1 low. That probably beats your pullup.

Which page in the refrence manual??

 

It's scattered around the schematics. Look for "MEMS_INT1".

The Discovery boards are kinda fun, but the gizmos eat up a lot of pins. Nucleo boards are more versatile.

 

I found that some STM32f429zi discovery board has some io pins that are not free to use as general io purpose because they are used with other on board chips , how can i determine these pins and how to determine the free ones !!!

 

Download the User Manual and study the circuit schematic.

https://www.st.com/resource/en/user...t-with-stm32f429zi-mcu-stmicroelectronics.pdf

 

Download the User Manual and study the circuit schematic.

There's also Table 6, which is quicker to scan once you figure out what you're looking at.