why I2C communication pull up resistor needed?

[praveen4u83]

My question:

Why open drain PINS are used for I2C communication. Why not normal I/O pins are used for I2C communication.

 

[tpetar]

Both SCL and SDA lines are "open drain" drivers. What this means is that the chip can drive its output low, but it cannot drive it high. For the line to be able to go high you must provide pull-up resistors to the 5v supply.

Using the I2C Bus
https://www.robot-electronics.co.uk/acatalog/I2C_Tutorial.html

 

[praveen4u83]

Both SCL and SDA lines are "open drain" drivers. What this means is that the chip can drive its output low, but it cannot drive it high. For the line to be able to go high you must provide pull-up resistors to the 5v supply.

Using the I2C Bus
https://www.robot-electronics.co.uk/acatalog/I2C_Tutorial.html

Thank you for the reply.I understand about open drain pins.My question is why open drain pins are used for I2C communication,why cant normal I/O pins used for I2C communication.If we use normal I/O pins pull ups are not required right,controller can make it high or low.

 

[tpetar]

How will you change value of that builtin resistor in IC epoxy body and adjust value to project needs ?

I2C Translator Pull-up Resisters: Internal vs. External?
https://www.fairchildsemi.com/colla...or-Pull-up-Resisters-Internal-vs-External.pdf

Effects of Varying I2C Pull-Up Resistors
**broken link removed**

 

[ech]

Thank you for the reply.I understand about open drain pins.My question is why open drain pins are used for I2C communication,why cant normal I/O pins used for I2C communication.If we use normal I/O pins pull ups are not required right,controller can make it high or low.

The SDA pin of all devices are connected together on the bus. Ths use of 'normal i/o' could lead to short circuit if one device drives high and another drives low.
With open drain, multiple output can be connected together.

 

[FvM]

I understand about open drain pins.My question is why open drain pins are used for I2C communication,why cant normal I/O pins used for I2C communication.

Apparently you do not understand. Open drain drivers are required by design of the protocol.

If we use normal I/O pins pull ups are not required.

If you use normal (push-pull) I/O-pins, the I2C bus doesn't work, at least for SDA. SCL can be push-pull driven by the master under certain prerequisites.

 

[chinlin0924]

Apparently you do not understand. Open drain drivers are required by design of the protocol.


If you use normal (push-pull) I/O-pins, the I2C bus doesn't work, at least for SDA. SCL can be push-pull driven by the master under certain prerequisites.

but why big-banging works for GPIO (normal I/O pins) ?

please take a look at this link:

https://en.wikipedia.org/wiki/Bit_banging

 

[FvM]

but why big-banging works for GPIO (normal I/O pins) ?

Did you notice that all usual processor GPIO pins can be operated as open drain output? That's what we do in a "bit-banging" I2C interface.

The dedicated I2C pins of most processors are also general IO pins, by the way.

 

[chinlin0924]

Did you notice that all usual processor GPIO pins can be operated as open drain output? That's what we do in a "bit-banging" I2C interface.

The dedicated I2C pins of most processors are also general IO pins, by the way.

do you mean normal (push-pull) I/O-pins and GPIO are not the same?


Regards,
Chinlin

 

[FvM]

GPIO are capable of push-pull operation but not restricted to it. For the details, you should refer to specific processor hardware documentation.

 

[chinlin0924]

GPIO are capable of push-pull operation but not restricted to it. For the details, you should refer to specific processor hardware documentation.

sorry! my confuse is as on this statement "If you use normal (push-pull) I/O-pins, the I2C bus doesn't work".
Is GPIO pin normal I/O pins? If yes, by this statement, the I2C bus doesn't work?

Thanks

 

[FvM]

The term "normal I/O pin" has been introduced by the OP, he also associated it with driving "high or low", in other words push-pull operation. So besides all ambiguousities of terms, the question is rather clear and has been clearly answered by several contributors.

 

[chinlin0924]

The term "normal I/O pin" has been introduced by the OP, he also associated it with driving "high or low", in other words push-pull operation. So besides all ambiguousities of terms, the question is rather clear and has been clearly answered by several contributors.

Can you help to post where I can find the discussion?
It seems a bit confuse on the term definition "normal I/O", "general I/O" with/or without open drain support.