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I am working on a project where I have three different power inputs. The first two are programming headers (for a msp430 and atmega328) and the third is a battery input. All three will power the entire system when plugged in, but the battery is disabled when either of the headers are plugged in.

My circuit is showing some strange behavior. Firstly, I am able to program the MSP430 through its programming header and blink a test LED. This draws about 60 mA due to the power MUX, diode, and voltage regulator. Everything is fine at this point. My confusion comes when I plug in the battery without either of the programming headers. Nothing happens, no LED blinks. I've tested all the points along the battery path and there is about 3 volts on the output of the voltage regulator..but the power supply (simulating a battery) shows no current draw, or at least less then 1 mA so I can't see it. So it would seem the MSP is not receiving sufficient current to turn on. However, I soldered a test lead to the output of the regulator and was able to light an external LED. So I know the regulator is working correctly. All grounds are common throughout the board.

In my testing, I found that with the battery input there is a voltage drop of about .7 V from the input of the regulator to its output. With the MSP header there is less then .1 V drop across it.

So my question is, why is there such a large voltage drop across the regulator when the battery is plugged in but not when the MSP header? Where is this mysterious impedance coming from? This is my second version of this board and I did not run into this problem on the first.

To clarify, the IC on the left is the power mux for the two programming headers. Its output leads to the input of the regulator (buck converter) which shares a node with the battery, as shown.

I'd appreciate any ideas or comments. Thanks!

enter image description here

Edit: RST in wired like this: enter image description here

So RST doesn't have a connection from the battery. I am confused though because in my first revision of this design, I used the exact same diagram for RST and it definitely works with the battery.

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  • \$\begingroup\$ What voltages do you measure for Vcc, the battery, and from the MSP header? \$\endgroup\$
    – Steve G
    Mar 31, 2016 at 1:12
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    \$\begingroup\$ How do you have RST wired? If it's not held high, the MSP430 won't run. \$\endgroup\$
    – Passerby
    Mar 31, 2016 at 3:56
  • \$\begingroup\$ It may be your code, did you turn the RST pin as a input or anything the first time around? Or are you using a different msp430? Some have internal pull-ups on RST. And measure the voltage at RST while on battery for the first board and the new one. Is R3 populated? Pin 2 on the header should be VCC, unless you don't have that wired to the rest of the VCC rail. \$\endgroup\$
    – Passerby
    Mar 31, 2016 at 18:55

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It turns out the issue is because RST was not pulled high. The first board didn't have this issue because I tied SRC_MSP directly to VCC.

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