ATmega16 datasheet(156/315 Pages) ATMEL

156 / 315 page

156

ATmega16(L)

2466E–AVR–10/02

Read Access

Doing a read access to the UBRRH or the UCSRC Register is a more complex opera-

tion. However, in most applications, it is rarely necessary to read any of these registers.

The read access is controlled by a timed sequence. Reading the I/O location once

returns the UBRRH Register contents. If the register location was read in previous sys-

tem clock cycle, reading the register in the current clock cycle will return the UCSRC

contents. Note that the timed sequence for reading the UCSRC is an atomic operation.

Interrupts must therefore be controlled (for example by disabling interrupts globally) dur-

ing the read operation.

The following code example shows how to read the UCSRC Register contents.

Note:

1. The example code assumes that the part specific header file is included.

The assembly code example returns the UCSRC value in r16.

Reading the UBRRH contents is not an atomic operation and therefore it can be read as

an ordinary register, as long as the previous instruction did not access the register

location.

USART Register

Description

USART I/O Data Register –

UDR

The USART Transmit Data Buffer Register and USART Receive Data Buffer Registers

share the same I/O address referred to as USART Data Register or UDR. The Transmit

Data Buffer Register (TXB) will be the destination for data written to the UDR register

location. Reading the UDR Register location will return the contents of the receive data

buffer register (RXB).

For 5-, 6-, or 7-bit characters the upper unused bits will be ignored by the Transmitter

and set to zero by the Receiver.

Assembly Code Example(1)

USART_ReadUCSRC:

; Read UCSRC

r16,UBRRH

r16,UCSRC

ret

C Code Example(1)

unsigned char

USART_ReadUCSRC( void )

{

unsigned char

ucsrc;

/* Read UCSRC */

ucsrc = UBRRH;

ucsrc = UCSRC;

return ucsrc;

}

Bit

5432

RXB[7:0]

UDR (Read)

TXB[7:0]

UDR (Write)

Read/Write

R/W

Initial Value

0000

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