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\nTTL Serial Communication Protocol Explained | Part 3\n

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\n\nPublisher:\n\nProgramming Electronics Academy\n\n\n
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\nLet’s look at a concrete example. The ASCII character capital G is represented by the bits 0100 0111. If we were to monitor the logic levels of the TX pin on the Arduino UNO while we are printing a capital G using Serial.begin() and...\nLet’s look at a concrete example. The ASCII character capital G is represented by the bits 0100 0111. If we were to monitor the logic levels of the TX pin on the Arduino UNO while we are printing a capital G using Serial.begin() and Serial.print(), void setup() { Serial.begin(9600); } void loop() { Serial.print(\"G\"); } it would look something like this… Serial communication packet So what’s going on here? THE START BIT You’ll notice that at the start of the transmission, the logic level is HIGH. This is the default logic level of the TX pin when not sending a message. When the sender is ready to send information, it starts by bringing its TX pin LOW – this is the start bit. The receiver is on the lookout for this and when it sees the logic level go from HIGH to LOW on its RX pin, this state change tells the receiver “Hey, a message is about to come in!”. The Receiver, based on the baud rate, then begins to read in the message one bit at a time that is coming in at its RX pin. The Receiver waits one and a half cycles to start sampling the line after it sees the initial state change. Why does it wait one and a half cycles, and not just one cycle? Well, by waiting 1 and a half cycles, it is able to sample right in the middle of the incoming signal. So it can be sure what it reads is the correct signal. Say you are pounding nails into studs to hang drywall – you want to get your nail right into the center of the stud to get the best quality connection. The receiver wants to sample right in the middle of the transmission to get the best guarantee that what it measured was the true signal. THE MESSAGE AKA DATA FRAME Referring back to the diagram above, now the Receiver is actually reading in the message. The message portion of the transmission is called the data frame. The first time it samples, the logic level is HIGH. If you imagine the Receiver being some guy with a clipboard, he’d be like, “Hey, let me mark a 1 down.” The next time the Receiver samples, it’s still HIGH. That’s another 1. After that, it’s still HIGH, that’s another 1. Then it goes LOW, the Receiver would read that in as a 0. And this keeps up for 8 bits. You’ll notice that the order of bits in the capital G is reversed – this is because the serial protocol sends the least significant bit first. This is how it was done long ago to confuse people like me. Well OK, I’m sure there is some valid reason for sending the least significant bit first, but hey – it’s still confusing even when you know it. THE PARITY BIT Next comes an optional parity bit. The default serial communication protocol for Arduino using Serial.begin() does not send a parity bit, but we’ll talk about it shortly, because I think it is pretty neat. The term parity refers to whether a number is even or odd. If you asked me the parity of the number 12, the answer would be even. Having a parity bit is a way for the receiver to check if the information received in the data frame was corrupted – that is, parity bits are for error checking. When a parity bit is used, both the sender and receiver agree ahead of time on either even or odd parity. The parity is determined by counting the number of bits with the value 1 in the data frame including the parity bit. Now I know that sounds super confusing, but it is deceptively simple. Say for example, the sender and receiver agree to have an even parity. This means all the 1’s in the data frame PLUS the parity bit need to equal an even number. So if the data frame has three 1s, then the sender will set the parity bit to a 1 as well – this way, the total number of ones is 4, which has even parity.\n
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8:09
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Higher Ed
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STEM
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\nProgramming Electronics Academy\n\n
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Date
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2021
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English
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For Teacher Use
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transmission, time, electronics
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arduino, arduino tutorial, arduino lesson, open source hardware group, learning arduino, arduino ide, arduino sketch, c++, programming electronics academy, electronics basics, electronics basics for beginners, electronics basics great scott, serial communication, electronics technician, electronics engineering, arduino startups, computer programming for beginners, logic level, transmit pin, bod rate, baud rate, data frame, significant bit, serial begin, start bit, parity bit, receive pin, total number, start, parity, signal, receiver, number, stop, send, capital, bits, message, high, sender, computer, notice, sending
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