Read Access

Doing a read access to the UBRRH or the UCSRC Register is a more complex operation. 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 system 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)

   ; Read UCSRC
   in r16,UBRRH
   in r16,UCSRC
unsigned char USART_ReadUCSRC( void )
   unsigned char ucsrc;
   /* Read UCSRC */
   ucsrc = UBRRH;
   ucsrc = UCSRC;
   return ucsrc;
Note: 1. See About Code Examples.

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.