So I'm using an arduino to control an MPR121 (MPR) capacitive sensor, two DRV2605L (DRV) haptic motor boards, and an H-bridge to control two solenoids. I got a TCA9543A I2C multiplexer (TCA) from digikey so I could switch between the two DRV boards as they have a fixed I2C address. I calculated the pullup resistors for the SDA/SCL lines per the TCA data sheet, which was 440-2950 ohms, I used 1Kohm resistors for the pullups for the I2C pins and used 570 ohm resistors for the interrupt and reset pins. All the pullups are connected to 5V. I placed a 100 nF capacitor between the TCA Vcc and GND as a bypass filter.
When connected only with USB, I was able to switch between the two DRV boards and activate the respective motors, but I could not switch both motors on at the same time. When I added the DC supply source either by itself or with the USB, I could detect the TCA and open a switch to detect a DRV with an I2C scanner script, but when I tried to write to the DRV to activate a motor, the motor would not activate. To reiterate all the I2C lines have a 5V 1Kohm pullup and the DRV and TCA operate up to 5.5V.
Addendum: I am using an arduino Uno R3, I need to use the DC supply to power the solenoids, my code runs as expected if the DC supply is not connected. Should I x-post this to the I2C forum?
