I have come across the following problem a few times in the past:
You have a break-out board where some pins have already been pulled up or down for convenience, e.g. RESET pulled high to keep the board running even when you don't connect the input pin to your MCU. Now you want the opposite standard behaviour when the pin is not set (e.g. in sleep mode or when no power is applied to the MCU). How can you achieve this? The only answer I can think of is to (physically) remove the pull-up and hook a pull-down between the MCU and the break-out board. I would rather not mess with the SMDs on the break-out board, so is there another solution?
A concrete example is an audio amplifier that is by default on and needs to be pulled down by a pin to switch off. Now when I remove power from the MCU, it automatically turns on again and pops audibly.
Yours gratefully
Jerome
Best Answer
The best way is (as you suggest) to remove the pull-up from the PCB and fit a pull-down elsewhere. But, you could try just fitting a pull-down resistor of much lower value than the pull-up. The two resistors will form a voltage divider, but with one resistor much lower in value than the other, the lower value will "win" and pull the pin down. The disadvantage of doing this is higher power consumption then the MCU pulls the pin HIGH. Make sure the pull-down resistor does not draw more current from the MCU pin than it can supply (check the datasheet) when pulling HIGH.