2017-06-25 05:00 AM
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-resistorSolved! Go to Solution.
2017-06-26 07:29 AM
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.
2017-06-26 07:04 AM
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.
2017-06-26 07:29 AM
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.
2017-06-27 03:40 AM
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.