Configuring Multiplexed Pin PWR# [1]/I2C_SDA in HX2VL - KBA218370

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Question:

How do we use the multiplexed pin PWR# [1]/I2C_SDA in HX2VL?

 

Answer:

After power-on-reset, the I2C_SDA becomes active to boot from the externally-connected EEPROM and acts as an I/O pin. After enumeration, this will switch to Power Enable (PWR#) pin of the power switch for DS port power control, which is the output pin. Because I2C_SDA requires a pull-up resistor as per the I2C specification, PWR# pin should be Active Low.

It is not recommended to use Active High power enable switch, because it requires a pull-down resistor. Therefore, the combination of pull-up for I2C_SDA and pull-down for PWR results in different logic level on the specified pin, leading to malfunction; for example, the power switch will always be ON and cannot be used for switch functionality. 

Note: For 28-pin packages, this pin can be used as either PWR# or I2C_SDA, but not as both. If EEPROM is connected, then the pin acts as I2C_SDA and will not switch to PWR# mode.