Questions about the PCA954X I2C bus switch

[Chromate]

    I'm currently using the I2C switch PCA9545 and PCA9548 in my design, and I have several questions about the output pin.

    1) Are the SDn/SCn output pins open-drained? Judging from the necessity of pull-up resistors, we assume they are open-drained, but no evidence can be found in the datasheet.

    2) In the datasheet, the voltage translation is explained by an instance, in which the upstream bus has the highest voltage (5V) among all of the downstream buses (3.3V and 2.7V), and the design guide demands that the PCA954X should be powered with the lowest I2C bus voltage(2.7V) . My question is whether it is okay when upstream voltage is lower than (and furtherly, the lowest in) all of the downstream buses? For instance, if the SDA/SCL input is 1.8V, and all SDn/SCn is 3.3V, should we power the PCA954X switch with VDD=1.8V?

[ErikaC]

Hello,

Yes, they are open-drain, pull-up resistors are REQUIRED on all upstream and downstream channels.

Yes, you should power the PCA954X switch with VDD=1.8V. The multiplexers/switches should be supplied with the lowest I2 C voltage needed to ensure proper voltage translation.

Please find more information here: https://www.nxp.com/docs/en/application-note/AN262.pdf

Hope this helps.

[Chromate]

Thank you for your reply!

According to the PCA9545 datasheet, the operating power supply range is 2.3 V to 5.5 V, so is it okay to power it at Vdd=1.8V?

[ErikaC]

Hello,

Yes, you are right, please use the VDD within the limits.