Lecture 10: Communications protocols

Question 1

What does USART stand for?

Answer 1

Universal Synchronous and Asynchronous Receiver and Transmitter

Question 2

What is USART?

Answer 2

It is a highly flexible transmission device. It has a configurable transmission speed and data format with error checking.

Question 3

In a USART, how do we implement 0 and 1?

Answer 3

using different voltages that depend on the connector standard.

Question 4

What is the BAUD rate?

Answer 4

It is the speed of transmission and represents the number of bits sent per second.

Question 5

How do we connect two USARTS?

Answer 5

Connect the transmitter of one device to the receiver of the other and vice versa.

Question 6

What is a frame?

Answer 6

It is how we send data. One frame will contain the data bits to be generated along with bits to start and stop transmission. There is an optional parity bit for error checking.

Question 7

What happens when one frame has been transmitted?

Answer 7

We can transmit a new frame or we can send the line into an idle state.

Question 8

What is the value of the start bit?

Answer 8

Low

Question 9

What is the value of the stop bit?

Answer 9

High

Question 10

What happens if our parity bit does not match the computed bit?

Answer 10

This means that the data is incorrect and it will be discarded.

Question 11

How does the Parity bit work?

Answer 11

We add it to the end of our data bits to make the number of High bits even or odd.

Question 12

How do we calculate the Parity bit?

Answer 12

Through the XOR of all the data bits.

Question 13

What is the location of the Parity bit?

Answer 13

between the last data bit and the stop transmission bit.

Question 14

What are the 5 steps for transmission?

Answer 14

Set the idle bit to high.
Set the start bit to low
Set the bits according to the data being sent
Set the Parity bit
Set the stop bit to high/

Question 15

What happens when we activate USART transmission?

Answer 15

The normal port operation is overridden and the function is given to the transmitters serial output.

Question 16

How do we initiate data transmission?

Answer 16

We load the transmission register with the data to be sent.
Move the data to a shift register when it is ready to send a new frame.
Once complete the frame will be transferred at the set clock rate.

Question 17

When will the receiver begin to receive data?

Answer 17

When it recognises a valid start bit

Question 18

What happens to the received data?

Answer 18

It will be received at the baud rate and transferred into a shift register.

Question 19

What are the 3 interrupts of the receiver?

Answer 19

Frame Error: This will detect out of sync conditions
Data Overrun: Indicates loss of data because the receiver is full
Parity Error: The Parity bit does not match

Question 20

What happens if we have incorrect baud rates between devices?

Answer 20

Then we will not correctly transmit the data and end up with weird results.

Question 21

What are the four clock modes that we can use for generation and transmission?

Answer 21

Normal or double speed asynchronous

Master | Slave synchronous

Question 22

What does SPI stand for?

Answer 22

Serial Peripheral Interface

Question 23

What is SPI?

Answer 23

It allows for high speed data transfer between the ATMega8 and its peripherals.

Question 24

How many data registers does the SPI consist of?

Answer 24

2 shift registers and a clock generator.

Question 25

How do we initiate the communication cycle?

Answer 25

The Master will pull low the slave select pin on the desired slave. Both Master and Slave will then begin to prepare the data for transmission in their respective registers and the Master will generate the required clock pulses.

Question 26

What is the MOSI line?

Answer 26

It is the Master Out Slave In line and is the line that data is always shifted on for data going from the Master to the Slave

Question 27

What is the MISO line?

Answer 27

It is the Master In Slave Out line and data from the Slave to the Master will transmit along this line.

Question 28

What happens between the Master and Slave after each data packet is sent?

Answer 28

The master will synchronise the slave by pulling high its ss bit.

Question 29

What is TWI interface?

Answer 29

It is a Two Wire serial interface that is suited for microcontroller programs.

Question 30

How many devices can we connect on the TWI Line?

Answer 30

128 as it has a 7 bit address space.

Question 31

What external hardware do we need for the TWI?

Answer 31

An external pull up resistor for each of the bus lines.