LPC2148 JTAG pin configuration

Started by Sutton Mehaffey February 20, 2008
I have pullups on my JTAG lines (works fine), but noticed that in the
pin definitions on page 73 of the user manual that the JTAG pins
already have pullups on them. Is this when they are defined as GPIO
or do they have them when configured as JTAG? If I don't really need
them, I want to remove my 10K pullups from my board.

Sutton

An Engineer's Guide to the LPC2100 Series

Well... There is the empirical way to find out if it is true or not.
Since a logic 1 for the LPC2148 (ViH) is about 2 volts, and you assume
that a 10K pullup goes to a +3.3volt rail. Remove your 10K pullup from
one of the JTAG pins, then, assuming a voltage divider would be needed
to GND on that pin, put a 20K resistor from the pin to GND. Then
measure the voltage. You should see about 2.2volts there.

That should give you about 110ua of drain on that pin.

TomW