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Lecture 2.2: UART Synchronization

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The Arduino Platform and C Programming

Universidad de California en Irvine

4.6 (3,138 calificaciones) | 90K estudiantes inscritos

Curso 2 de 6 en An Introduction to Programming the Internet of Things (IOT) Programa Especializado

The Arduino is an open-source computer hardware/software platform for building digital devices and interactive objects that can sense and control the physical world around them. In this class you will learn how the Arduino platform works in terms of the physical board and libraries and the IDE (integrated development environment). You will also learn about shields, which are smaller boards that plug into the main Arduino board to perform other functions such as sensing light, heat, GPS tracking, or providing a user interface display. The course will also cover programming the Arduino using C code and accessing the pins on the board via the software to control external devices. Please note that this course does not include discussion forums. Upon completing this course, you will be able to: 1. Outline the composition of the Arduino development board 2. Describe what it means to program the board's firmware 3. Read board schematics 4. Install Arduino IDE 5. Describe what "shields" are and how they are used 6. Specify the role of libraries in the use of shields 7. Compile and run a program 8. Name C Variables and Types 9. Name common C operators 10. Use conditionals and loops 11. Explain functions, their definition and invocation 12. Explain the implications of global variables 13. Undertake the Arduino build process 14. Describe the role of the tools behind the IDE 15. Describe how to invoke functions in classes 16. Explain the structure of an Arduino sketch 17. Access the pins of the Arduino 18. Differentiate between digital and analog pin 19. Debug embedded software 20. Explain the importance of controllability and observability in the debugging process 21. Describe common debugging architectures for embedded systems 22. Explain how the UART Serial communication protocol works 23. Describe how the Arduino Serial library performs serial communication

Destrezas que aprenderás

Arduino, C Programming, Debugging

Revisiones

4.6 (3,138 calificaciones)

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RS

Oct 02, 2017

Nice course. The platform and programming is explained in a simple way. If you are already a C or C++ programmer, only the topics dealing with the Arduino platform will be important.

VP

Dec 11, 2017

This course is the basic for starting with Arduino or Internet of Things.\n\nHere it has been taught in a very simple language.\n\nGreat efforts by the professor Ian Harris.

De la lección

Module 4

This module is an introduction on debugging embedded software on an Arduino. We discuss the basic debugging requirements: controllability and observability. The debugging environment available for an Arduino UNO is limited, so we describe how to use the UART communication protocol to gain controllability and observability. We present the use of the Serial library to communicate with the Arduino through the serial monitor.

Lecture 2.1: UART Protocol7:37

Lecture 2.2: UART Synchronization7:45

Lecture 2.3: UART Parity and Stop6:26

Impartido por:

Ian Harris
Professor

[MUSIC]

We're continuing to talk about UART protocol and

we're gonna talk right here about how synchronization happens.

So the transmitter and the receiver, they have to be synchronized.

That is the receiver has to know when to expect data from the transmitter.

And it has to be fairly accurate about the timing that it expects.

And UART is an asynchronous protocol meaning it has no clock.

Normally, if you have a synchronous protocol,

the clock is how it's synchronized.

When it sees the rising edge, it knows data will be sent.

But this doesn't have a clock.

So it has to figure out when data is gonna be sent another way.

So UART synchronization.

So what it means is that the receiver has to know when it's

looking at that value on that wire, that serial wire.

It has to know which bit it's reading.

If it's suppose to read and what bit it's reading.

It has to count the bits.

So for instance say, it knows that you're sending 8 bits.

It has to know okay now,

is when I should sample bit number one and then now is when bit number two happen.

Now is when bit number three should happen and so on.

So it has to know when to expect each bit to arrive, so

it know when to sample the signal on that wire to get a 1 or a 0.

If it gets out of sync like that, that's synchronized.

If the receiver knows when to expect each bit sent from the transmitter.

If it's not synchronized properly then

the receiver will not expect the bits at the right time.

It'll receive the wrong data.

So here's an example.

Right here we're show a little timing diagram.

And we're really showing three bits being sent.

There's a 1, there's a 0 and there's a stop bit.

That's what should be sent.

That's what the transmitter is actually sending.

Now a stop bit is high, the stop bit is at the end where the signal is actually high.

So it should be sending a 1 then a 0 then a stop bit.

These are just the last bits of communication.

So I'm just showing the bits 7 and

8 of the last two bits of say an 8 bit set of data.

And then after the last two bits, you're gonna expect to stop bits.

So what should be, the expected bits as they are correctly observed would be a bit

7 would be 1, bit 8 would be a 0 and then the stop bit is the next bit and

the signal is high and it would say oh okay, this is a stop.

And if that happened, if the receiver correctly knew when to observe

those signals, it would receive the correct bits.

It would receive a 1 for bit 7, a 0 for the bit 8.

And then it will receive high at the stop bit and that means that it's working.

That means that is correct.

That is a correct assumption.

The stop bit has to be high.

If the stop bit is low then there's a problem with the communication.

So if the receiver knows exactly when to sample the signals,

it will receive the right bits.

But let's say the receiver is off, it is not synchronized properly.

It started sampling earlier, too early.

So if it started sampling too early, it might sample the values at the wrong time.

So when that one comes through instead of thinking oh,

this is bit 7, it might think it's already on bit 8.

And if that were the case then when that 0 comes through, instead of thinking it's

bit 8, it would think that that was the stop bit since it's the one after bit 8.

And then it would read a 0 for the stop bit and

that cause a failure because a stop bit has to be 1.

So if it was not synchronized right then it could read a 0 for

the stop bit, that would cause a failure and will loose the whole byte and

that byte would have to be resent.

So what I'm saying here is that the receiver has to be synchronized with

the transmitter.

It has to know when the transmitter is gonna send a byte, a bit.

It has to know which bit is being sent.

So it needs to synchronize on the beginning of the communication.

So that's what the start bit is for.

So imprecise start time.

If it gets the start time wrong then it can send the wrong,

it'll receive the wrong bits and maybe fail in its communication.

So the start bit is how synchronization happens.

So the start bit remember, at the beginning before you've actually started

sending anything, the wire is high.

The single wire is gonna be high.

The receiver knows its transmission is gonna start.

The start bit is happening when it goes from high to low.

So when there's a falling edge on that signal then it says okay,

this is the start bit.

Now is when I have to start synchronizing myself.

Now, we've got two examples.

Two pictures up here, it's like timing diagrams.

And what they're showing is two different situations.

Now remember that this UART, it was made a long time ago.

It's made to be robust in the face of noise.

So noise, you get all kinds of electromagnetic noise on these signals.

So this signal maybe it's supposed to be high because you're not communicating but

there's some kind of glitch.

Some kind of noise,

some kind of electromagnetic noise which forces the signal to go low, mistakenly.

Now when that happens, if the receiver is too quick, it might say oh,

the signal went low, I guess communication is starting but maybe it's just a glitch,

a temporary bounce down.

And it shouldn't consider that to be a real signal.

It should ignore a short glitch.

So what happens is, it's gonna measure the time that the signal is low.

So on the left, it's just a glitch.

The signal goes low but for a very short time.

If it's too short then the receiver should say oh, that's not real,

that's not really a start.

But on the other hand on the right, if this signal goes low and stays low for

a significant amount of time then the receiver should say oh,

this is a real start I need to synchronize myself.

So the receiver has to be able to distinguish between a glitch for

a start which is not a real start signal and a real start.

So it hs to count how long it stays low.

Now see those little up arrows on the timing diagrams?

Those are the sampling points.

Those are the points in time where the receiver is checking the signal value.

And you'll notice with the glitch, it goes low for only three sample sample points.

So the receiver would say oh, it's five or rather low for three sample points.

That's not enough.

Where if you look at the one where the startup is detected,

the one on the right, it's low for I don't know eight sample points and

it would say okay, that's efficient.

So that's what's gonna happen on the receiver end.

It's gonna sample over and over, faster than the baud rate.

Typically 16 times faster at least, and it's gonna count how many times

to find the start bit, it's gonna count how many samples it's low for.

If it's low for enough samples then it says yes, that is a real start.

So detection of the start bit is used to synchronize the receiver and

the transmitter and it's synchronized based on the falling edge

of the signal and it recognizes the start bit based on the falling edge.

The following 0 must be of sufficiently long duration

to screen out any kind of noise.

The receiver has to sample faster than the baud rate, typically 16 times faster.

That's common.

The start bit Is indicated by a 0 for at least half a period.

So the length of the period depends on the length of your baud rate.

But let's say, you're using a baud rate of 9600 baud.

If you remember, the period length of that is 104 microseconds.

So the hold period is 104 microseconds, half a period would be 52,

52 microseconds.

So the receiver, if it sees that signal go down load for

52 microseconds then it says okay, this is a real start signal.

We're really starting communication,

I need to synchronize myself now against that falling edge.

Where if it's less than 52 microseconds then it says oh, that's just a glitch,

I can ignore that.

Thank you.

[MUSIC]

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