TCA9545A: Issue accessing the device.

Kevin Lai said:

Part Number: TCA9545A

Hi my customer is having an issue accessing the device on their design.

the application is 1.8V master talking to multiple 3.3V slaves that share the same I2C address. they are not getting an ACK back when they send the 0x70 address to the mux, but they do have the address pins tied low. 

also, the datasheet states the Vpass must be equal to or lower than the lowest bus voltage. is the lowest bus here 1.8V or does this only apply to the slave bus side?

would they have to change VCC to be higher, say 2.5V?

[Bobby] The problem appears that the customer's Vcc is 3.3V based on the schematic from your email. ViH of the device is now 0.7*3.3V = 2.31V. This means his 1.8V rail will never satisify the ViH of our device. I would suggest the customer change the Vcc to 1.8V instead.

additionally, is there any issue with pulling the REST pin low through a resistor? the idea was to ensure the mux wasn't incorrectly enabled. they drive this pin to 1.8V to enable it. is this too low? should they remove the pull-down resistor and tie the RESET pin to a higher VCC than 1.8V?

[Bobby] That is not a problem. Using a resistor to GND just provides a weaker bias (its an RC constant). The HIGH needs to satisfy the ViH for the reset pin. Typically this is 70% of Vcc. I suggest that they change Vcc to 1.8V.

Please send me a direct email to continue the discussion with more details.

Thanks,

Kevin

Comments in bold above.

-Bobby