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Output push-pull with pull-up or pull-down capability

tzk
Associate II

The output push-pull implies either 0 or 1 - never floating. When do we use the output push-pull with pull-up or pull-down capability? What is the application for the output push-pull with pull-up or pull-down configuration?

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Accepted Solutions
Andreas Bolsch
Lead II

Well, for a pin statically configured as push-pull output none. But if the pin is dynamically reconfigured, i.e. switched from output to input, it's handy not to have to switch pull-up/down on and off every time. Saves time, flash and ... headaches. As the order of switching mode and pull-up/down would matter. And if the pull-up/down would be automatically disabled when switching to output, the precise timing would be critical.

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6 REPLIES 6
raptorhal2
Lead

Check the data sheet of any devices connected to your GPIOs. That should provide any pull up or pull down requirements. For example, SPI chip select usually requires a pull up to place the device in standby at power on while the MCU is getting the other pins configured.

AScha.3
Chief

 output push-pull implies either 0 or 1 - ok. thats it.

>What is the application for the output push-pull with pull-up or pull-down configuration?

its only useful for input , or open collector out + pullup . all other is nonsense. 

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Andreas Bolsch
Lead II

Well, for a pin statically configured as push-pull output none. But if the pin is dynamically reconfigured, i.e. switched from output to input, it's handy not to have to switch pull-up/down on and off every time. Saves time, flash and ... headaches. As the order of switching mode and pull-up/down would matter. And if the pull-up/down would be automatically disabled when switching to output, the precise timing would be critical.

Thanks. What you say matches this blog article: 

https://resources.pcb.cadence.com/blog/are-pull-up-resistors-needed-on-push-pull-buses
I am working on the device required to have the pull-up. The device does not need to be reconfigured dynamically. But then, it works with or without a pull-up resistor setting. And some of my team members think what AScha.3 describe: 

"its only useful for input , or open collector out + pull-up. all other is nonsense. "

So, under what situation will we be seeing this pull-up requirement? How can I test it out so the device fails without configuring the pull-up setting?

 

Thanks. It makes sense. 

LCE
Principal

Basics of EE, learn what a push-pull output means, then you will see that any pull-up or down resistor makes no sense at all - the only exception: you want to have some minimum load and burn some calories. 😉