ATmega48/88/168
USART MSPIM Baud Rate
Registers - UBRRnL and
UBRRnH
The function and bit description of the baud rate registers in MSPI mode is identical to
normal USART operation. See “USART Baud Rate Registers – UBRRnL and UBRRnH”
on page 184.
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.
A comparison of the USART in MSPIM mode and the SPI pins is shown in Table 88 on
page 197.
Table 88. Comparison of USART in MSPIM mode and SPI pins.
USART_MSPIM
TxDn
SPI
MOSI
MISO
SCK
SS
Comment
Master Out only
RxDn
Master In only
XCKn
(Functionally identical)
Not supported by USART in MSPIM
(N/A)
197
2545D–AVR–07/04
ATMEL [ ATMEL ]
