[MUSIC] So an I2C transaction, structure of a transaction, means the sequence of zeros and ones that you expect to see on the two lines. The data line and the clock line. In order to send data, to write data or read data, okay? I'm gonna go over what the transaction looks like in detail over time. So what ones and zeros sequence is expected on those two lines. So one thing to note is that in I2C, every transaction is either a write transaction, meaning the master is writing to the slave, sending data to the slave. Or it's a re-transaction, meaning the master is gonna receive data from the slave. So, every transaction starts off with a start condition. A start condition is a condition on the two signals, the SDA and SCL that indicates the beginning of the transaction. I'll show you that on the next slide. So the start condition is meeting, once the start condition happens the master creates the start condition on the bus. And then the slaves are all listening to the bus. And they look for the start condition, when they see the start condition they sensibly perk up their ears, They start listening because they know a transaction is about to start. So, [COUGH] after the start condition is sent, then you gotta send the address. So every slave on this bus has an address, a known address. And the addresses are seven bits long. There's also another variance to this protocol where addresses can be ten bits long, but right now let's say seven bits long. So you got this seven bit long address that's associated with every slave and every slave knows their own address. So the master, when it's sending data to a particular slave, starting a transaction with a particular slave. It has to send that seven bit long address on the wires, one clock cycle at a time. On an SDA line, say the address is 0000001. So it puts that sequence on the SDA line. One clock cycle at a time, one SCL. So the first SCL clock, you'll have a zero. Next one, you'll have a zero, and so on. It puts them on one at a time. And then, in addition to the address, there's one more bit that is a direction, okay? So all together, the address and direction make up a byte, that's eight bits all together. So seven bit address plus a one bit direction. And the direction is, it's just a single bit, zero or one. It indicates whether this is a read transaction or a write transaction. So what happens is, at the point when the address and the data direction are being sent, the slaves are all listening for the address. They receive the address bits one at a time. Then once they receive all seven, they compare those address bits to their own address. And if it's a match, then they continue to interact on this transaction. And if it's not a match, then all the rest of the slaves are aware it's not a match. They just go to sleep and forget about this transaction and wait until it's over. Okay, so once you've sent the address and the direction, then there's a data byte or a set of data bytes that can be sent. Now, if this is a right transaction, then you send as many data bytes as you wanna write. You wanna write ten bytes, you send ten data bytes. If this is a read transaction, then what happens is the slave has to send bytes back, as many bytes as are needed to be read. So there's a sequence of data bytes that happen, and data bytes keep getting transferred until finally, there's a stop condition. The message says okay, communication over. And it sets a stop condition, sends that on the data line, the SDA line. And the slave recognizes that, and then stops the transmission. So that's the structure, that just to look at the start of this, the beginning of this, we talked about a start condition. So a start condition and a stop condition, here's what they look like. This is how you start and stop any transaction. So there are two lines, SDA and SCL. The SDA line, if it falls, if there is a falling edge on that line. So you can see the SDA line is high. It goes from high to low. If that happens while the SCL line, the clock line, is high, then that's a start condition. Now, stop conditions is the reverse. The SDA line goes from zero to one while the clock line, the SCL line, is high. So that's a stop condition. So when the master wants to start a communication, they, the master drives SDA from high to low on wall SCL. Serving SCL high and if it wants to stop the communication, then it just does the opposite. It drives SDA from low to high while the SCL is high. So start condition is falling transition on SDA while SCL=1. And stop condition is a rising transition on SDA while SCL=1. Thank you. [MUSIC]
