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 (e.g., by disabling interrupts globally) during the
read operation.
The following code example shows how to read the UCSRC Register contents.
Assembly Code Example(1)
USART_ReadUCSRC:
; Read UCSRC
in r16,UBRRH
in r16,UCSRC
ret
C Code Example(1)
unsigned char USART_ReadUCSRC( void )
{
unsigned char ucsrc;
/* Read UCSRC */
ucsrc = UBRRH;
ucsrc = UCSRC;
return ucsrc;
}
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
Bit
7
6
5
4
3
2
1
0
RXB[7:0]
TXB[7:0]
UDR (Read)
UDR (Write)
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
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.
150
ATmega8(L)
2486M–AVR–12/03
ATMEL [ ATMEL ]
