Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 206
BAUD Baud rate (in bits per second, bps)
f
OSC
System Oscillator clock frequency
UBRRn Contents of the UBRRnH and UBRRnL Registers, (0-4095)
21.4 SPI Data Modes and Timing
There are four combinations of XCKn (SCK) phase and polarity with respect to serial data, which are
determined by control bits UCPHAn and UCPOLn. The data transfer timing diagrams are shown in Figure 21-1.
Data bits are shifted out and latched in on opposite edges of the XCKn signal, ensuring sufficient time for data
signals to stabilize. The UCPOLn and UCPHAn functionality is summarized in Table 21-2. Note that changing
the setting of any of these bits will corrupt all ongoing communication for both the Receiver and Transmitter.
Figure 21-1. UCPHAn and UCPOLn data transfer timing diagrams.
21.5 Frame Formats
A serial frame for the MSPIM is defined to be one character of 8 data bits. The USART in MSPIM mode has two
valid frame formats:
 8-bit data with MSB first
 8-bit data with LSB first
A frame starts with the least or most significant data bit. Then the next data bits, up to a total of eight, are
succeeding, ending with the most or least significant bit accordingly. When a complete frame is transmitted, a
new frame can directly follow it, or the communication line can be set to an idle (high) state.
The UDORDn bit in UCSRnC sets the frame format used by the USART in MSPIM mode. The Receiver and
Transmitter use the same setting. Note that changing the setting of any of these bits will corrupt all ongoing
communication for both the Receiver and Transmitter.
16-bit data transfer can be achieved by writing two data bytes to UDRn. A UART transmit complete interrupt will
then signal that the 16-bit value has been shifted out.
Table 21-2. UCPOLn and UCPHAn Functionality
UCPOLn UCPHAn SPI Mode Leading Edge Trailing Edge
0 0 0 Sample (Rising) Setup (Falling)
0 1 1 Setup (Rising) Sample (Falling)
1 0 2 Sample (Falling) Setup (Rising)
1 1 3 Setup (Falling) Sample (Rising)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
XCK
Data setup (TXD)
Data sample (RXD)
UCPOL=0 UCPOL=1
UCPHA=0
UCPHA=1