TCA9548A: TCA5948A over 8CH drive

Hey Katsuhiro,

I just want to clarify what you mean by "Customer IC has only 2 I2C address, they want to use 10 ICs(10CH)"

Do you mean they have 10 devices but each only has 1 bit to program address?

Example: (0x20h and 0x21h)

"So by using eight TCA9548A, is it possible to extend to 64CH ?"

-Yes, this is possible but can present capacitance issues as you place more in parallel because you also add up capacitance.

By using several  TCA9548A, is it any limitation of I2C speed by SDA, SCL capacitance?

It depends.

Speed: Rise times can be affected if you have a lot of capacitance of the bus and will depend on the pull up resistor value (smaller is faster but uses more power). A slow rise time can affect I2C speed. Propagation delay with this device is very low and should not present issues so it won't affect speed much.

capacitance: This will likely be what you will be concerned with. Each SDA/SCL pin of TCA9548A have about 20~28pF of capacitance and 5.5~7.5pF for each switch channel. If we add in the transceivers at around 10pF, you are looking at:

2 TCA9548A SDA/SCL: 28x2=56pF

1 TCA9548A 8Ch: 7.5x8=60pF

1 TCA9548A 2Ch: 7.5x2=15pF

10 devices 1 Ch each: 10x10= 100pF

Total estimated capacitance= 231pF

This is the capacitance due to everything on TCA9548A and does not include the other devices which may be on the I2C bus and the master's capacitance. This alone looks like it would be okay with 100kHz I2C limit of 400pF max.

^If the devices have the same voltage level and you can place 2 on one channel, then we can cut these numbers down further to one TCA9548A and half Ch used.

Ex.

Master<-----I2C Bus---->TCA9548A<-----Ch0----->device1(0x20h) and device2(0x21h)

                                                         <-----Ch1----->device1(0x20h) and device2(0x21h)

                                                         <-----Ch2----->device1(0x20h) and device2(0x21h)

                                                         <-----Ch3----->device1(0x20h) and device2(0x21h)

                                                         <-----Ch4----->device1(0x20h) and device2(0x21h)

-Bobby