Pull up/down resistors on PCB?

[Simon Andersen]

Posted on June 25, 2017 at 14:00

I am designing a PCB which uses the STM32F446 MCU. I have the first revision done and working. But I am wondering if all the pull-up and -down resistors on the board can be excluded and use the internals instead?

The board has the following components connected with pull up/down:

->Micro SD card in SPI mode

->User button

->UART connection with other board

When should you use external resistors and when can you rely on the internals?

Thank you for any thoughts on the topic.

#pull-up #pull-down #pcb-guidelines #pull-up-resistor

[Zt Liu]

Posted on June 26, 2017 at 16:04

I don't pull up for SPI interface,

Internal pull up/down for Button is enough.

As for UART....I think it's only necessary for external pull up if it's 1 wire half-duplex case.

[misagh · Accepted solution]

Posted on June 26, 2017 at 16:29

I, personally, use external pull up/down resistors in cases which the pull up/down resistor should be known and an specific value. In other cases like your cases, it would be great to use internal pull up/down resistors. 

[Simon Andersen]

Posted on June 27, 2017 at 12:40

I have found that most of the external pull up/down resistors can be excluded. However, when working with the SD-card, it is advisable to use an external pull-up resistor for the chip select, to avoid sending bad information to the card during initialization. From this, i have concluded that anything that needs specific voltage levels before and during initialization, or needs a specific value, should have external resistors. Else, i will use the external resistors instead.