6. Communication

8/3/2019 6. Communication

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Microcontrollers.

Communication:AVR ATmega32Joel Castillo Gmez


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Communication

Another important feature of microcontrollers is the ability to

communicate with other devices:


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USART

To do this the microcontroller has a protocol called USART (Universal

Synchronous and Asynchronous serial Receiver and Transmitter)

Its main features are:

- Full Duplex Operation (Can receive/transmit at the same time)- Asynchronous or Synchronous

- High Resolution Baud Rate Generator (minimum error)

- 5 to 9 Data Bits, 1 or 2 Stop Bits and Odd or Even Parity

- 2X Speed Asynchronous Comm Mode

- 3 Interrupt sources. (Tx Complete, Rx Complete, TxReg[UDR] Empty)

We will review its characteristics and setup.


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USART

USART Structure

The Data:

UDR

Configuration & Status Registers:

UCSRA UCSRB UCSRC*

Error Detection /

Double Speed /

Multi-processor

Mode

Receive Enable /Transmit Enable

(RXEN, TXEN)

Receive PinRxD

(PD0)

Transmit Pin

TxD

(PD1)

Baud Rate:

UBRRH*

UBRRL

Ext. Clock

Pin - XCK

(PB0)

Status Flags

(Bits 7, 6 y 5)

RXC, TXC, UDRE

Interrupt Enable:

(Bits 7, 6 y 5)

RXCIE, TXCIE, UDRIE

Bit 8

(RXB8,

TXB8)

UCSZ2**

Register

Select*

(URSEL)

1=UCSRC

0= UBRRH

Mode

Select

(UMSEL)

0=Asynch.

Char Size

UCSZ1, UCSZ0 **ucsz2,ucsz1,ucz0=(0,1,0)

= 8 bit

Parity

Mode(UPM1,

UPM0)

Stop Bit(s)Select

(USBS)


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USART


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USART

Baud Rate Configurator

Config & Status Reg A

Config & Status Reg B

USART Data

Config & Status Reg C


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USART

How is the information sent/received in UDR?

UCSZ2, UCSZ1, UCSZ0

UPM1, UPM0

USBS

Configured trough:

Velocity or Speed of the Frame is

configured trough UBRRH & UBRRL

Note: Parity checks the

number or 1s in the Data.

In even parity, when thereare even number of 1s, the

parity bit is 0. The opposite

for odd parity.


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USART Configuration

Baud Rate Calculation:


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USART Configuration

Common Baud Rates @ 8Mhz:


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USART Configuration

USART mode Select:

Parity Mode:

Stop Bit Select:


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USART Configuration

USART Character Size:


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USART Error Checking

UCSRA USART Control and Status Register A

Inside this register we have 3 bits for Error Checking:

FE*: This bit is set when there is no STOP bit detected.

DOR: This bit is set when the buffer (2 characters in UDR + 1 char inthe receive Shift Register) are full, and at the same time there hasbeen a Start bit detected.

PE*: This bit is set when the Parity (if it is enabled) did not match atthe end of the reception.

* Note: This 2 bits apply to the next character to be read (in UDR)


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USART Asynchronous

We will review the Asynchronous Mode of operation to show

how to communicate with another device:

- In this mode, only Tx and Rx pins are needed (no XCK) since

there is no Synchronization.


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USART Asynchronous

So, if we wanted to communicate (just transmit) with another device @9600 bps and the frame format = (8 bits, Even Parity, 1 Stop bit):

1. [UCSRC] Set UMSEL properly to get Asynchronous mode:UMSEL = 0

2. [UCSRC & UCSRB*] Configure the frame format:UCSZ2*, UCSZ1, UCSZ0 = 3 or (0, 1, 1) -> 8-bits data. *Note, no need to write UCSRB

UPM1, UPM0 = 2 or (1, 0) -> Even parity.

USBS = 0 -> 1 Stop bit

3. [UBRRH & UBRRL] Calculate & Load the BaudRate:UBBR = (8,000,000 Hz) / (16 * 9600) 1 = 51.0833 OR

UBBR = (8,000,000 Hz) / (8 * 9600) 1 = 103.166

UBBR = 103 (Note: Real BAUD = 9615.38 bps & U2X = 1 in UCSRA) orUBBR = 51 (Note: Real BAUD = 9615.38 bps & U2X = 0 in UCSRA)

4. Remember to make URSEL = 1 in [UCSRC] in order to write to UCSRC.(Note: If URSEL bit = 0, the contents will be written to UBRRH)

5. [UCSRB] Finally, enable the transmit. [Note: To receive make RXEN = 1]TXEN = 1


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USART Asynchronous

Now that we have the configuration lets make the following

program:


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USART Asynchronous

Note that we do NOT

do any error checking,

since we are only

transmitting.


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USART Asynchronous

Activities:

- Configure the USART @ 1 Mbps (8 bits, odd parity & 1 stop

bit) & transmit an incrementing register every 100 ms (using

interrupts to count the 100 ms).


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Reference:

ATmega32(L) (Rev. 2503P-AVR-07/10)

Documents

6. Communication