STM32F042K6U6: Using PF11 as GPIO/EXTI Pin & Clarity required on Boot Process

[shashank26]

Dear Team,

As per the datasheet, the PF11 is used for BOOT0. However STM32CubeIDE doesn't specify the PF11 as BOOT Pin in the IOC File. 

Also, The Reference Manual doesn't specify usage of the BOOT Pin (Grey marked area).

Please provide clarity on the following,

1) How does the boot up process occur in this MCU? Does it require a resistor pull down at PF11 Pin for normal boot to main flash memory?

2) If not, can PF11 be used as normal GPIO/EXTI Pin?

Awaiting your response.

Regards,

Shashank

[TDK]

1) Per the table in the RM, if BOOT_SEL=1, then BOOT0 pin must be tied low to boot into flash. If BOOT_SEL=0, BOOT0 pin state doesn't matter.

2) The BOOT0 pin state is only sampled at startup. After that, you can configure it as GPIO and use how you want.

> Also, The Reference Manual doesn't specify usage of the BOOT Pin (Grey marked area).

What is unclear about the table? A "x" means "don't care" which means the pin can be in any state.

[shashank26]

Hi TDK,

Thank you for your response. I am a hardware engineer and would like to understand the requirements from a design perspective.

I have a few follow-up questions,

1) What is the default state of a blank chip?

2) While programming the chip for the first time, do I need to pull the boot pin to ground?

Regards,

Shashank 

[TDK]

1) What is the default state of a blank chip?

Flash is empty. Per the reference manual, BOOT_SEL=1, which means the state of the BOOT0 pin matters.

2) While programming the chip for the first time, do I need to pull the boot pin to ground?

If you are programming over SWD, which is typical, no, it will connect regardless. But to run the program on powerup after it's on there, BOOT0 will need to be low.

Generally, BOOT0 should always be tied to or pulled to ground at the hardware level.