Hi,
We are trying to interface acceleration sensor (3.3V) with Raspberry pi board through I2C and we need bus length to be 1meter or more. So, we want to use bus driver IC. We are unable to figure out which one to use from Bus Extender and Level shifting i2c bus repeater as both can serve the purpose. And we are trying to operate at fast mode of i2c communication(i.e., 400kHZ)
Please suggest which one to use.
Hey Jai,
"we need bus length to be 1meter or more."
What is the maximum distance you expect?
What kind of cable (Cat 5?)?
"We are trying to interface acceleration sensor (3.3V) with Raspberry pi board"
I assume the Raspberry pi is also using 3.3V for I2C.
From what you have provided if the cable distance isn't that long, we can probably use a standard I2C buffer to do this. I would say TCA9517 will do the trick.
Note: I omitted the pull up resistors in this diagram which are necessary to have.
To make sure this device works properly, you need to make sure B sides of this device are not connected to each other. I would also recommend you at least place a slot for a schottky diode on the board incase there is a huge negative overshoot generated due to the parasitic inductance of the cable. The schotkyy diodes will help limit the negative swing and prevent the device from getting damaged.
Thanks,
-Bobby
Jai,
Assuming the bus capacitance of around ~220pF (52 per meter(3), 10 per TCA9517(3x) and 10 per connection(3x) ). The equivalent resistance on the cable side needs to be about 1600 ohms max and 1000 ohms minimum so to be between these two values we can use three 3500 ohm resistors to give an equivalent resistance of 1166 ohms.
The pull up resistors between the master/slaves can be larger values up to about 11k assuming bus capacitance between the two to be about 30pF. I would just choose 3.5k ohms on these as well.
"Raspberry pi will have internal pull up resistors?"
-Likely it is 10k, because the cables are going to introduce prop delays I would suggests bypassing it would a stronger pull up resistor to try to minimize the delays.
My calculations above do not take into account the prop delays this kind of set up will introduce which may hinder your maximum frequency you can actually communicate at. (You may not be able to get away with 400kHz but may have to slow it down to 300k Hz for example.)
-Bobby
