topic Re: Internal Pull-Up Resistors in LPC Microcontrollers

Internal Pull-Up Resistors

lpcware — Wed, 15 Jun 2016 19:33:58 GMT

Content originally posted in LPCWare by cdoerr on Tue Jul 30 03:25:18 MST 2013
Hello,

According to the datasheet the LPC1114 (and other LPC microcontroller) have internal pull-up resistors on their GPIO pins. In some forum posts I have however read that these pull-ups are fairly weak. Hence my question: If I want to connect another IC (example below) to these GPIO pins, will the internal pull-ups be sufficient or will I need additional external resistors?

Thanks,
Christian

As an example take the MCP73871 charge IC. The datasheet says: "STAT1 is an open-drain logic output for connection to an LED for charge status indication. Alternatively, a pull-up resistor can be applied for interfacing to a host microcontroller."

Re: Internal Pull-Up Resistors

lpcware — Wed, 15 Jun 2016 19:33:59 GMT

Content originally posted in LPCWare by Wouter on Tue Jul 30 03:47:00 MST 2013
The pull-up is basically a constant current source, rated at typically 50µA (http://www.nxp.com/documents/data_sheet/LPC111X.pdf, table 16 on page 60).

Probably the pull-up is strong enough for your STAT1 pin example, though this depends on the spec of that specific pin (i.e. the HIGH-level input current for the STAT1 pin).

Re: Internal Pull-Up Resistors

lpcware — Wed, 15 Jun 2016 19:34:00 GMT

Content originally posted in LPCWare by cdoerr on Tue Jul 30 03:59:15 MST 2013
Thank you for the pointer, this was the number I was looking for.

In my MCP73871 example list a sink current of 16 mA and an input leakage current of max 1µA (http://ww1.microchip.com/downloads/en/DeviceDoc/22090b.pdf, page 6), so for this case the internal pull-up seems indeed enough.