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ATmega8 I2C(TWI)

Serial Communication Interfaces

The world of electronics is replete with communication protocols, each designed to cater to specific needs and applications. Among these, the I2C (Inter-Integrated Circuit) protocol stands out for its simplicity and efficiency.

Originally conceived by Philips Semiconductor in the 1980s, I2C was designed to facilitate communication between components in a television set. Over the decades, its utility has expanded manifold, finding applications in a multitude of devices and systems.

Atmega8 I2C

At its core, I2C is a two-wire, bi-directional serial bus that provides a communication link between multiple devices. These two wires, known as SDA (Serial Data) and SCL (Serial Clock), are the lifelines of the I2C protocol.

The beauty of I2C lies in its simplicity. With just these two wires, it's possible to connect and facilitate communication between a plethora of devices.

But what led to the inception of I2C? In the era of burgeoning electronics, there was a pressing need for an efficient, low-cost, and versatile communication method to connect ICs on a board without a plethora of wires. I2C was Philips Semiconductor's answer to this challenge.

Today, its applications span across embedded systems, sensors, memory devices, displays, and much more.

History and Evolution of I2C

The origins of I2C date back to the early 1980s. Philips needed a simple and efficient way to let their chips communicate with each other in televisions. This need led to the birth of the I2C protocol (also pronounced as "I squared C").

Over the years, I2C has seen multiple upgrades and enhancements, adapting to the demands of modern electronics. From its initial slow-speed mode to the introduction of high-speed modes, I2C has continued to be a favorite among engineers and hobbyists alike.

Fundamentals of I2C in ATmega8

The Two-Wire Serial Interface (TWI) is ideally suited for typical microcontroller applications. The TWI protocol allows the systems designer to interconnect up to 128 different devices using only two bi-directional bus lines, one for clock (SCL) and one for data (SDA).

The only external hardware needed to implement the bus is a single pull-up resistor for each of the TWI bus lines. All devices connected to the bus have individual addresses and mechanisms for resolving bus contention are inherent in the TWI protocol.

Bus interconnection of TWI(I2C)

tenancy Connecting Devices in I2C (TWI) Communication

When you're connecting multiple devices on an I2C bus, it's important to understand how they communicate. In I2C, there are two special wires: SDA (Serial Data) and SCL (Serial Clock). Here's how it works:

Pull-Up Resistors

Both SDA and SCL wires are connected to a positive supply voltage through something called "pull-up resistors." These resistors are like gentle helpers that keep the wires at a high voltage level when no one is actively using them. They ensure that the wires stay quiet and ready for communication.

Bus Drivers

The devices on the I2C bus, like sensors or microcontrollers, have a specific way of talking. They use something called "open-drain" or "open-collector" bus drivers. Imagine these drivers as traffic cops for the wires. They can only pull the wire down to a low level, not push it up to a high level.

Wired-AND Function

When multiple devices are connected and use these open-drain drivers, it creates a cool teamwork effect. If any device wants to make the wire low (like sending a '0'), it can do that, and it won't conflict with other devices trying to do the same thing.

It's like when a group of friends votes on something, and as long as one person says "no," the decision is "no."

Low and High Levels

So, when one or more devices on the I2C bus want to send a '0' (low), they can do it by pulling the wire down. But when nobody is pulling the wire down (all devices are quiet), the pull-up resistors gently pull the wire up to a '1' (high) level.

Important Note

To make this teamwork happen, all the devices connected to the I2C bus need to be powered up. It's like saying everyone in the room has to be awake and ready to participate for the discussion to happen.

Number of Devices

You can connect quite a few devices to an I2C bus, and the limit depends on two things: the total capacitance of the wires (which should stay below 400pF) and the addresses that devices use to identify themselves (7-bit addresses).

It's a bit like having multiple phones in a room, and each phone has its own unique number to call.

Data Transfer and Frame Format in I2C Communication

When you send information on an I2C bus, there are some rules to follow:

I2C Data validity

Transferring Bits

Each piece of data you send on the I2C bus is like a puzzle piece. To make sure everyone understands the puzzle piece correctly, there is a clock signal that acts like a metronome, ticking with each piece. When the metronome is high, the puzzle piece must be stable and not change.

There are only two times when the puzzle pieces can change: when you start a conversation (START) and when you finish it (STOP).

START and STOP Conditions

Imagine you're the leader of a conversation, like the host of a meeting. You begin the discussion by saying "START" and end it by saying "STOP." While the conversation is happening (between START and STOP), no one else should interrupt or try to take control of the talk.

Sometimes, in the middle of a discussion, you might want to say something new without ending the conversation. This is like a "REPEATED START." It's like saying, "Hold on, I have more to say." But remember, even during a REPEATED START, the conversation is still ongoing until you finally say "STOP."

You can signal START and STOP by changing the level of the SDA line (the data line) when the SCL line (the clock line) is high. Think of it as a way to say "Let's start talking" or "Okay, we're done talking."

Address Packet Format

Every time you want to talk to someone on the I2C bus, you need to use a special code. It's like dialing a phone number. This code is 9 bits long: 7 bits for the address, 1 bit to say if you want to "read" or "write," and 1 bit for acknowledgment.

When you call someone (address them), they need to say "hello" back (acknowledge). If they're busy or can't talk, they won't say "hello." In I2C, this is done by making the SDA line low during the ninth metronome tick (ACK).

If you can't reach someone, you can end the call (STOP) or start a new one (REPEATED START). The MSB of the data byte is transmitted first.

Data Packet Format

The actual information you send is like the message you're delivering in your call. Each piece of information is 9 bits long: 8 bits for the data and 1 bit for acknowledgment.

When someone gets your message (receives data), they acknowledge it by making the SDA line low during the ninth metronome tick (ACK). If they don't acknowledge, it's like they didn't receive the message (NACK).

Combining Address and Data Packets

A complete conversation (transmission) includes START, addressing (who you're talking to), data (what you're saying), and finally, STOP (ending the talk). Just START and STOP with nothing in between doesn't make sense.

If you're sending a lot of information, you can keep talking between the address and STOP, depending on what you and the other person agreed upon in advance.

Sometimes, you can slow down or speed up the conversation using the clock signal. It's like adjusting the pace of the discussion. The person receiving the information can extend the pause between the ticks if they need more time to process.

That's the basics of how information travels on an I2C bus - like a well-organized conversation with a clear beginning, middle, and end, where everyone knows when to speak and when to listen.

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