ATMEGA8_14 Datasheet, PDF (147/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

Read Access
ATmega8(L)
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
(for example, 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.
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