lpcware

LPC17xx weak pull up max. current?

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jul 29, 2016 by F W Johnson
Content originally posted in LPCWare by ntipping on Wed Nov 20 03:24:25 MST 2013
Hi All,

Apologies - I think I've answered my own question. Looks as though from the data sheet that it's a maximum of 85uA over temperature for a Vin of 0V - equivalent to about 39k. I'll leave the original question as the points I make are still valid, and I'd be interested in other views:

The LPC1769 data sheet only appears to have typical specs for the port weak pull ups. Does anyone have a maximum over temperature? I am in the middle of designing the LPC1769 into an existing application so I need to achieve compatibility with boards already in production. This is an ethernet-based modular controller which has up to 34 IO lines which can be inputs or outputs (depending on the boards fitted). The system expects the reset condition to have all outputs at 0. With all pins in reset defaulting to inputs (sensible) but with weak pull ups (not so sensible) the reset state of pins destined to become outputs is potentially 1. This is a no-no in a system controlling moving parts. I can't tolerate overriding pull downs on 34 pins (don't have space) and in any case, this would make a nonsense of a low power design. Could be a show-stopper on using NXP for me, especially if M4's (next project) behave the same. I don't think I'm the first to be caught out by this behaviour. My only hope would be if the max. current under all conditions is less than the min. drive required to turn an output on.

Best Regards,

Nick Tipping

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