Internal weak-pull up resistance value

[yoshiyukiokada]

How much is the resistance value of internal weak-pull up in LPC802 GPIO?

I'd like to keep a line which is connected to a MCU (LPC802) low level during it is in reset state. I believe that GPIO is configured as input function with internal weak-pull up resistance when the MCU is in reset state. Hear "reset state" means that the MCU's reset pin is low. Therefore, I consider placing an external pull-down resistance on the line and I'd like to know the resistance value of internal weak-pull up to calculate the appropriate external pull-down resistance value.

[kerryzhou · Accepted solution]

Hi Yoshiyuki Okada,

    Actually, the internal weak pull-up is the transistor, like the following picture:

The simulated resistance value is about tens of K Ohm.

    Wish it helps you!

Have a great day,
Kerry

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------

[ianbenton]

It's in the Datasheet Table 9 "Static Characteristics" Ipu Pull-up current. It is specified between 10µA and 85µA. It probably looks more like a constant current source, but as a resistor it somewhere between 39kΩ and 330kΩ.

Taking the worst case (85µA) and the maximum value for Vil (input low voltage of 0.3Vdd at 3.3V, which is 0.99V, then the largest resistor which will ensure pull-down is 0.99V/85µA = 11kΩ