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IO when msp430 is powered down?

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I've got a main board and then some external boards that all have msp430s in them. The main board communicates with i2c, using the hardware drivers. The main board can, using a regulator for each external board, turn off the power on each of the external boards.


But when I turn off the power on the external board, the external MSP430 (which is powered down) pulls down the I2C-bus. Is there some way I can leave the turned-off msp430 so that it won't mess up the i2c-bus it's connected to? 


The external msp430s are MSP430G2553 and the main board is an F5528. 

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You could switch all external peripherals on the external boards off, while leaving the MSP430s on the external boards in a low power mode (LPM4 for best power conserving).

This does mean you cannot switch off the external boards completely though.

Another option might be to an I2C buffering IC on the external boards (or the local board on the lines going to the external board) that will not affect the I2C lines when powered off.

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