What is the purpose of 33ohm serise resistor in I2C signal lines? and why 33ohm?

[winterice]

I see many circuits with I2C bus that there always have 33ohm series resistors on I2C signal lines: SDA and SCL , what is the purpose of that resistors? damping ? or termination? or something else???

and why 33 ohm?

could any bull help me?!

 

[dknguyen]

Is the 33 ohm resistor sitting between the signal lines of the circuits and the rest of the network (ie. between the device and before the junction where the lines split up to other devices?) If it is, then it's probably for protection to try and supress current spikes that may be induced on the line.

It's probably 33 ohms so it provides enough current limiting against spikes or transients but is low enough to allow the maintain the integrity of the square wave's rise and fall times.

You don't need those resistors there so it's probably for protection.

 

[winterice]

thanks for your response, dknguyen
While there are no other devices, just source and sink device, the 33ohm sit between source device and the pull-up resistor (2.2K).

Is the 33 ohm resistor sitting between the signal lines of the circuits and the rest of the network (ie. between the device and before the junction where the lines split up to other devices?) If it is, then it's probably for protection to try and supress current spikes that may be induced on the line.

It's probably 33 ohms so it provides enough current limiting against spikes or transients but is low enough to allow the maintain the integrity of the square wave's rise and fall times.

You don't need those resistors there so it's probably for protection.

 

[dknguyen]

It's for protection. The resistor is 33 ohms because it's small enough to not form a voltage divider with the pull-up resistor to affect the voltage thresholds of the I2C signal, but is still enough to protect the device from current surges in the I2C lines (which may be long).

 

[winterice]

that's interesting, thanks

It's for protection. The resistor is 33 ohms because it's small enough to not form a voltage divider with the pull-up resistor to affect the voltage thresholds of the I2C signal, but is still enough to protect the device from current surges in the I2C lines (which may be long).

 

[Eplanet]

hello winterice
may you show me that circuit for example.

 

[winterice]

yes of cause.