lauchpad P1.3 pull-down resistor

Hi, Paul.

Paul Medina said:

I have an old launchpad and a new launchpad. I usually use the push button of my old launchpad as an input for testing my programs. When I tried to do the same on my new launchpad I realized it didn't work well... After a while, and by chance, I enabled the P1.3 pull-down resistor and surprisingly the push button of my new launchpad worked in the same way that my old launchpad... Can somebody explain me why is this happening if the schematics connections seem to be the same in both?

The quick answer is that this is due to a LaunchPad hardware change.

The change (and others, including the new Rev 1.5 layout for jumper block J3) are described in Rev. B of the "MSP-EXP430G2 LaunchPad Experimenter Board User's Guide" (you want slau318b.pdf, the trailing "b" is important).

Section 1.3, "Revisions", describes the change: R34, the external pullup for P1.3, is no longer present on the Rev. 1.5 LaunchPad boards. This why you needed to change your code to enable the internal pullup for P1.3 to get the same effect.

Item 8 in Section 7, "Frequently Asked Questions" says, in part:

LaunchPad revisions 1.3 and 1.4 come with R34 populated. The 47-kΩ resistor is used as a pullup for the button S2. If the port P1.3 is driven to ground, as suggested to keep the power consumption down, the pullup resistor generates an additional current of approximately 77 μA. To reduce the power consumption, the port should stay in input mode or the resistor should be removed if button S2 is not used. The internal pullup of the MSP430G2xx can be used instead.

Hope this helps.

Frank