TCA9803: external pull-up at B side

Hi Fred,

As Clemen's pointed out, B side may not drive A side low. The worst case possibility is you may even see oscillations start to occur on the I2C bus. 

-Bobby

Hi Bobby,

I'm calculating the trace parasitic of my customer to evaluate if they need buffer, its total length is 21.5 inch with 5 mil width

what's the rule of thumb usually for 4 layer FR4 PCB?   5pF/inch or 7pF/inch?

would you show the equation and calculate it for me?

Fred,

It will depend on how the PCB is set up. 

You can find a calculator here:

https://technick.net/tools/impedance-calculator/microstrip/

This one assumes the trace is a microstrip. Permitivity of FR4 can vary, but I generally use 4.8 as the number. 

-Bobby

Hi Bobby,

I wonder how TCA9803 shorter rise time, so if it's A side master sending data, B side has internal 3.3mA current source, which make sense.

but what if B side slave sending data? there's only original external pull-up resistor at A side, no one boost the A side pull-up?

the R and Cbus at A side is still the same situation..

Correct, A side needs a pull up resistor to receive a rising edge/logic high. The TCA980x family is an I2C buffer/redriver meaning they separate the two sides from each other from a capacitive stand point. If you used a PCA9306 for example, it does not redrive/buffer so you would have the same capacitive loading shared on both sides when the signal is low and transitioning high. With the TCA980x, you only have to worry about the capacitive loading on their respective sides. side A will only have capacitance on A side and side B will only have capacitance on B side, they are never shared. This is what an I2C redriver/buffer does, it separates capacitive loading. It doesn't mean that both sides get an internal current source.

I2C standard has a capacitive loading limit of 400pF for standard mode and fast mode. 

-Bobby

While the A side does support 400 pF, you can design you circuit so that the A-side segment is smaller and most devices are on the B side. (I²C is bidirectional; it does not matter where the master is.)

If your total capacitance is below 400 pF and you do not need level shifting, you can leave the A side powered down and connect everything to the B side.

Hi 

so the know-how is all about placing buffer at the right location to distribute the Cbus,

if the total Cbus is 600pF,

if I place it just right next to the master, B side is still 600pF, so it would not help at all, right?