MSP430G2101: Won't initialize when power on P1.6

Andrew Baird

Part Number: MSP430G2101

I'm attempting to implement the following circuit. It's powered by 9v, which is passed through a momentary switch, and a voltage divider to bring the voltage down to ~2.7 volts. This voltage is used to enable a voltage regulator that is also supplied by the 9v.

When the controller initializes, it sends 3.3v to the voltage regulator to keep the power on after the button is released. It also turns on the led, to indicate that everything is working.

The issue i've encountered, is that i'm trying to send the 2.7v from the switch to P1.6, so that i can detect when the power button is pressed (The purple line in the schematic). When P1.6 is connected, the controller fails to initialize, and the voltage supplied to the enable pin of the voltage regulator is only 0.86v. When P1.6 is not connected, everything initializes correctly.

I'm sure i'm missing something obvious, but i'm not sure what to search for. Any help would be greatly appreciated.

Here is the code flashed to the controller

#include <msp430.h>

int main(void)
{
WDTCTL = WDTPW | WDTHOLD;
P1SEL = 0x00;
//P1.6 Input
P1DIR = ~BIT6;
//Output 3.3v on P1.1
P1OUT = BIT1;
P2SEL = 0x00;
P2DIR = 0xFF;
P2OUT = 0x00;
//Enable resistor on P1.6
P1REN |= BIT6;

//Turn on LED
P2OUT |= BIT6;
}

9 months ago

0 Andrew Baird 9 months ago

Prodigy 10 points

I should also point out, i have boards that to the best of my knowledge are using this exact circuit and work. I don't however have access to the code they're flashed with, so i don't know what they're doing.

+1 Keith Barkley 9 months ago in reply to Andrew Baird

Guru 37015 points

What you are missing are the ESD diodes and the Absolute Maximum ratings.

The absolute Max on a pin is Vcc + 0.3 V or so. When Vcc is 0, anything over 0.3 V on a pin exceeds the absolute Max. The practical effect is that the ESD diode on the pin is forward biased with respect to the VDD pin and current flows into the VDD rail, turning on the chip.

Most of us, like you, learn this the hard way.

0 Andrew Baird 9 months ago in reply to Keith Barkley

Prodigy 10 points

Now that you say that, I recall reading about the Absolute Maximum ratings at one point. You wouldn't happen to know where those are documented would you? I couldn't find it referenced in slau144k.

0 Keith Barkley 9 months ago in reply to Andrew Baird

Guru 37015 points

In the datasheet SLAS695I (As linked at the top of this discussion)

+1 JD Crutchfield 9 months ago in reply to Keith Barkley

TI__Mastermind 27240 points

Hey Andrew,

Just as Keith already pointed out sounds like you applying power to the GPIO first which is violating's the max ratings and potentially back-powering the MCU.

Thanks,

0 Andrew Baird 9 months ago in reply to JD Crutchfield

Prodigy 10 points

I'm certain you're both correct. The thing that is confusing me is that i have a board with this circuit, and it's not suffering from this issue. Both the 2.7v from the switch, and the 3.3v from the regulator rise at the same moment. And the board initializes as expected.

On the board that's failing, the protection diodes trigger immediately, and the voltage from the switch only rises to about 0.8v. The regulator is never enabled.

I could probably add a transistor that feeds the output from the voltage regulator into pin 1.6, and is driven by the switch voltage, but i would really like to figure out how this other board was made to function.

0 Andrew Baird 9 months ago

Prodigy 10 points

Looks like it was just the voltage regulator i'm using. I stole the voltage regulator from the working board, and the circuit started working.

0 JD Crutchfield 9 months ago in reply to Andrew Baird

TI__Mastermind 27240 points

Should have used a TI regulator!

Just kidding, I'm glad you found your root cause.

Thanks,

0 Andrew Baird 9 months ago in reply to Keith Barkley

Prodigy 10 points

I was looking more for documentation that talked about the ESD diodes. I was able to find that documentation here slaa689b

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MSP430G2101: Won't initialize when power on P1.6