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jeybop

Problem with high impedance state

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Hello,

 

I have an hardware question. I am testing multiplexing with my msp430g2231. To do that, I need three states, so I use the high impedance of a pin, setting it to input (P1DIR = 0)

 

It works like a charm with almost all pins: the hi-Z state produce an output of 1.5V

 

But on pins 0 and 1, hi-Z state shows 3V. Some unwanted LEDs weakly light on...

 

Is it a known issue? Or is there a problem with my msp430g2231?

 

Thanks!

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Are you connecting to the launch pad? is pin 1 and 0 p1.1 and p1.0 on the launch pad?

P1.0 on the launch pad is connected to an led and 1.1 is connected to the transmit of the uart. maybe removing the jumper for rtx and the led (assuming you are using a launch pad)

 

just a shot in the dark here.

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... the hi-Z state produce an output of 1.5V ...

 

 

hi-Z state does not "produce" any output voltage level. You probably did not connect it to anything and leakage caused it to "float" to 1.5V.

 

You should always use something else to pull that signal to either ~3V or ~0V. Never allow it to sit at ~1.5V.

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I think JMLB is right in his assessment and solution. You're probably getting a ground through the LED and/or the programming side of the LaunchPad. Disconnecting the P1.0 jumper at J5 and the TXD jumper at J3 should fix it (you'll need to put the TXD jumper back to reprogram the MSP430).

 

You should always use something else to pull that signal to either ~3V or ~0V. Never allow it to sit at ~1.5V.

He's trying to multiplex, or likely more specifically, charlieplex since he states he needs three states, so he's using the high-impedance of the input mode of a pin as the third state. Using a pull-up or pull-down would negate the ability to charlieplex.

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Are you connecting to the launch pad? is pin 1 and 0 p1.1 and p1.0 on the launch pad?

P1.0 on the launch pad is connected to an led and 1.1 is connected to the transmit of the uart. maybe removing the jumper for rtx and the led (assuming you are using a launch pad)

just a shot in the dark here.

 

You were absolutely right! More precisely, the guilty pins were P1.1 and P1.2. Removing RXD and TXD solved the problem!

 

He's trying to multiplex, or likely more specifically, charlieplex since he states he needs three states, so he's using the high-impedance of the input mode of a pin as the third state. Using a pull-up or pull-down would negate the ability to charlieplex.

 

Yes, that's what I am trying. And it is actually working quite well. MSP is like a drug!

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