Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 206
clock. The data written to UDRn is moved from the transmit buffer to the shift register when the shift register is
ready to send a new frame.
Note: To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must be read once for
each byte transmitted. The input buffer operation is identical to normal USART mode, i.e. if an overflow occurs the
character last received will be lost, not the first data in the buffer. This means that if four bytes are transferred, byte
1 first, then byte 2, 3, and 4, and the UDRn is not read before all transfers are completed, then byte 3 to be received
will be lost, and not byte 1.
The following code examples show a simple USART in MSPIM mode transfer function based on polling of the
Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be initialized
before the function can be used. For the assembly code, the data to be sent is assumed to be stored in Register
R16 and the data received will be available in the same register (R16) after the function returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with
new data to be transmitted. The function then waits for data to be present in the receive buffer by checking the
RXCn Flag, before reading the buffer and returning the value.
Note: 1. See “About code examples” on page 17.
20.6.1 Transmitter and Receiver Flags and Interrupts
The RXCn, TXCn, and UDREn flags and corresponding interrupts in USART in MSPIM mode are identical in
function to the normal USART operation. However, the receiver error status flags (FE, DOR, and PE) are not in
use and is always read as zero.
Assembly Code Example
(1)
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
sbis UCSRnA, UDREn
rjmp USART_MSPIM_Transfer
; Put data (r16) into buffer, sends the data
out UDRn,r16
; Wait for data to be received
USART_MSPIM_Wait_RXCn:
sbis UCSRnA, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
in r16, UDRn
ret
C Code Example
(1)
unsigned char USART_Receive( void )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}