207
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
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.
20.6.2
Disabling the Transmitter or Receiver
The disabling of the transmitter or receiver in USART in MSPIM mode is identical in function to
the normal USART operation.
20.7
AVR USART MSPIM vs. AVR SPI
The USART in MSPIM mode is fully compatible with the AVR SPI regarding:
Master mode timing diagram.
The UCPOLn bit functionality is identical to the SPI CPOL bit
The UCPHAn bit functionality is identical to the SPI CPHA bit
The UDORDn bit functionality is identical to the SPI DORD bit
However, since the USART in MSPIM mode reuses the USART resources, the use of the
USART in MSPIM mode is somewhat different compared to the SPI. In addition to differences of
the control register bits, and that only master operation is supported by the USART in MSPIM
mode, the following features differ between the two modules:
The USART in MSPIM mode includes (double) buffering of the transmitter. The SPI has no
buffer
The USART in MSPIM mode receiver includes an additional buffer level
The SPI WCOL (Write Collision) bit is not included in USART in MSPIM mode
The SPI double speed mode (SPI2X) bit is not included. However, the same effect is achieved
by setting UBRRn accordingly
Interrupt timing is not compatible
Pin control differs due to the master only operation of the USART in MSPIM mode
Table 20-3.
Comparison of USART in MSPIM mode and SPI pins.
USART_MSPIM
SPI
Comment
TxDn
MOSI
Master Out only
RxDn
MISO
Master In only
XCKn
SCK
(Functionally identical)
(N/A)
SS
Not supported by USART in MSPIM