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The the NXP LPC17xx Cortex-M3 family has internal pull-up resistors on some important pins such as the JTAG, NMI, Bootloader-select pin etc.

While reading the data-sheet I learned that the pull-up is internally connected to a voltage source of 2.3V to 2.6V. The core itself runs at 3 to 3.3V.

I see that 2.3V is good enough to give a reliable logic high level, but why didn't NXP just pulled up all these pins to VCC instead?

What is the advantage in doing so? NXP clearly didn't did it that way just for fun..

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  • \$\begingroup\$ A guess would be that the high voltage (3.3V) is only present at the input to the on-chip regulator, not distributed around the chip, and the pullup is to the internal core voltage, which I don't see specified. \$\endgroup\$ – Spehro Pefhany Mar 7 '16 at 16:03
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Edit:

The only reason I can think of is to provide a weak pull-up so a wide variety of controllers can manipulate the pins with open drain switching as it's very common. enter image description here

Also the Pull's are disabled for the TCK

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  • \$\begingroup\$ Yes, sure. It's UM10360.pdf: nxp.com/documents/user_manual/UM10360.pdf On page 101 you'll see the TCK pin along with the footnote [5] which indicates the pull-up to 2.3 to 2.6v. \$\endgroup\$ – Nils Pipenbrinck Mar 7 '16 at 15:29
  • \$\begingroup\$ This isn't an answer and you should delete it - leave it as a comment under the question. \$\endgroup\$ – Andy aka Mar 7 '16 at 15:39
  • \$\begingroup\$ It wouldn't let me comment.. need 50 reputation?? \$\endgroup\$ – Mr.Invisible Mar 7 '16 at 15:40
  • \$\begingroup\$ Just added a snap shot from the user manual that shows the TCK pin in interest. \$\endgroup\$ – Mr.Invisible Mar 7 '16 at 16:07

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