topic Re: External pull-down for LPC1114F Standard I/O in LPC Microcontrollers

External pull-down for LPC1114F Standard I/O

shimizu_hiroshi — Wed, 19 Apr 2023 07:37:22 GMT

I would like to have an external pull-down resistor on the Standard I/O pin of the LPC1114FHN33.

The internal pull-down resistance and pull-up resistance can be switched by setting the register, but this is because we want to fix the logic to "L" until the pull-down or pull-up is reflected.

So I have a question.

Please let me know the range of external pull-down resistor value that does not damage the LPC1114 when the supply voltage is 3.3V.

Best regards,

Hiroshi Shimizu

Re: External pull-down for LPC1114F Standard I/O

CarlosGarabito — Thu, 20 Apr 2023 05:11:21 GMT

The pull-up and pull-down are not passive resistors but active current sources (they typically deliver around 50µA when enabled) that is why there is not a precise value for them. Please refer to the "Ipu"/"Ipd" and "Vi" characteristics in the LPC11U2x data sheet (Electrical pin characteristics, located at the Table 5, page 31):

So for example, if a pin at pull-down has a typical Vi of 5V, the current would be of 50µA and therefore the internal resistor value would be of 100 KOhms.

I hope this information can help you, and please ask if you have any more doubts.

Re: External pull-down for LPC1114F Standard I/O

shimizu_hiroshi — Sun, 23 Apr 2023 23:37:25 GMT

Thank you for the information about internal resistance values!!

However, what I want to know the most is the allowable range of resistance when an external pull-down resistor is attached.

Anyone have any information regarding that?

Re: External pull-down for LPC1114F Standard I/O

CarlosGarabito — Tue, 25 Apr 2023 02:10:42 GMT

You need to calculate the resistance according to the characteristics of your MCU, as explained above.