Electrical – Open drain in I2C

i2copen-drain

In I2C, why SCK and SDA should be open drain? What will happen if they are not open drain. I am trying to implement I2C communication using bit banging. Should I enable Open drain circuitry of GPIO pins for SCk and SDA?
I am asking this may be because I dont know about open drain logic.

Best Answer

I²C is not a strict master-slave protocol (although your circuit might use it this way), but allows multiple masters, and allows slaves to delay transactions by clock stretching.

This means that multiple devices can drive the same signal line at the same time. And if two devices try to drive different levels, the result would be bus contention, which would not only make the actual level undefined, but also would form a short between V_CC and GND which could burn out the output transistors of these devices.

In an open-drain system, if one device pulls the bus low while another device lets it float high, the result is a low signal. This is used by the I²C protocol to detect conflicts in a multi-master system, and to let the master detect when a slave pulls the clock low to delay it.

If you have only a single master, and if your slaves do not support clock stretching, you can use a push/pull output for the clock line. But the data line is written and read in the same transaction, so you must never actively drive it to the high level.

Related Solutions

Electronic – PIC18F2550 I2C Open Drain

You can do open drain on any digital pin on the PIC yourself, it's easy! I'll show through a quick chunk of example code using RB0.

_TRISB0 = 1; // Set the pin to high impedance
_LATB0 = 0;  // Set the output low (this would be _PORTB0 on some pics)
// As long as you're using pin RB0 as a open-drain, don't touch _LATB0

// To output a "low" (drain) do this:
_TRISB0 = 0; // Set the pin to output, it's already low, so it will "drain"

// To output a "high" (open) do this:
_TRISB0 = 1; // Set the pin to high impedance, the pullup resistor will pull high

The I2C module will do something similar to this internally, without you doing anything. In other words: yes! You'll need a pullup resistor!

EDIT: To answer your second question which I did not directly address, the pin does not need a pullup resistor during normal digital output operation. My example was to show you how a normal, fully functioning digital I/O pin could operate as a open-drain without any additional behavior or behaving unexpectedly. I should also note that I have done this exactly what I showed here to do open-drain in real programs, so this isn't a toy example, it's really how you do it!

Electronic – Open drain concept in I2C bus

Is my adding of the red MCU correct?

Not exactly. The MCU is just another member of the bus like the rest of the Devices. There is nothing particularly special about how the MCU operates on the I2C bus. The MCU can recognize when either line is high or low, and the MCU can pull on either (or both) of the lines itself in order to communicate with all the other devices on the bus.

The bold part the chip is the red MCU, right?

It implies that the MCU is somehow different than the other devices on the I2C bus. But in reality, it is NOT special.

Why the chip cannot drive it high?

If you understood the nature of passive pullup, open-drain busses, you would understand that is not a proper question. You don't WANT any of the devices to drive it high because that would completely preclude any communication on the bus. The devices can only communicate if they can pull the bus LOW so that all the other devices on the bus can recognize that activity is taking place.

What does the open drain mean in plain English?

It is just like the signaling system used on many pubic transportation vehicles. There is a long "rope" along the length of the car, and ANYONE along the line can pull down on the "rope" to signal that they want to get off at the next stop. If you fixed the "rope" so that it was always HIGH, then nobody could pull on it to signal a stop.

Best Answer

Related Solutions

Electronic – PIC18F2550 I2C Open Drain

Electronic – Open drain concept in I2C bus

Related Topic