ATMEGA8_08 Datasheet, PDF (153/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

ATmega8(L)
Read Access
USART Register
Description
USART I/O Data
Register – UDR
Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. How-
ever, 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 system clock cycle, read-
ing 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. See “About Code Examples” on page 8.
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 ordi-
nary register, as long as the previous instruction did not access the register location.
Bit
Read/Write
Initial Value
7
R/W
0
6
R/W
0
5
R/W
0
4
3
RXB[7:0]
TXB[7:0]
R/W
R/W
0
0
2
R/W
0
1
R/W
0
0
R/W
0
UDR (Read)
UDR (Write)
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 Reg-
ister (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.
The transmit buffer can only be written when the UDRE Flag in the UCSRA Register is set. Data
written to UDR when the UDRE Flag is not set, will be ignored by the USART Transmitter. When
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