BOOT0 pull-down resistor value for STM32F469 and STM32F412

[Hitatachipoo415256]

Hello, 

We are using STM32F469 and STM32F412 in our design. 

Regarding the BOOT0 pin, we understand that a pull-down resistor is required to ensure boot from Flash. However, we could not find a clearly specified resistor value in the datasheet or application notes. 

In many reference designs, a 10k ohm resistor is used. In some cases, lower values such as 510 ohm are used. 

Could you please clarify the design criteria for selecting the BOOT0 pull-down resistor value? 

Is there a recommended range?

What factors should be considered, for example noise or leakage current?

Why are lower values such as 510 ohm sometimes used? 

We would like to have a clear technical justification for our design.

Thank you.

Hiroshi Yamada

[mfgkw]

The examples in DS11139 (B.2, B.3) suggest 10k. This is obviously low enough to pull down reliably.

Any lower value increases the current without need, but will not hurt besides of that.

[Peter BENSCH]

@Hitatachipoo415256 You can also connect the BOOT0 pin directly to GND, but that would no longer allow the pin to be explicitly set to 1. If you use 510Ω, then when setting it to 1 a correspondingly large current would flow, which is probably undesirable and is probably what @mfgkw meant.

Since it is a CMOS input, practically no current flows out of or into BOOT0, so, as also mentioned just now, around 10kΩ is a common value, which also helps to attenuate any small interference picked up (BOOT0 is only queried during a very short time window at power-up anyway).

Regards
/Peter