179
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
duces a two CPU clock period delay and therefore the maximum external XCKn clock frequency
is limited by the following equation:
Note that f
osc depends on the stability of the system clock source. It is therefore recommended to
add some margin to avoid possible loss of data due to frequency variations.
19.4.4
Synchronous Clock Operation
When synchronous mode is used (UMSELn = 1), the XCKn pin will be used as either clock input
(Slave) or clock output (Master). The dependency between the clock edges and data sampling
or data change is the same. The basic principle is that data input (on RxDn) is sampled at the
opposite XCKn clock edge of the edge the data output (TxDn) is changed.
Figure 19-3. Synchronous mode XCKn timing.
The UCPOLn bit UCRSC selects which XCKn clock edge is used for data sampling and which is
be changed at rising XCKn edge and sampled at falling XCKn edge. If UCPOLn is set, the data
will be changed at falling XCKn edge and sampled at rising XCKn edge.
19.5
Frame formats
A serial frame is defined to be one character of data bits with synchronization bits (start and stop
bits), and optionally a parity bit for error checking. The USART accepts all 30 combinations of
the following as valid frame formats:
One start bit
5, 6, 7, 8, or 9 data bits
No, even, or odd parity bit
One or two stop bits
A frame starts with the start bit followed by the least significant data bit. Then the next data bits,
up to a total of nine, are succeeding, ending with the most significant bit. If enabled, the parity bit
is inserted after the data bits, before the stop bits. When a complete frame is transmitted, it can
be directly followed by a new frame, or the communication line can be set to an idle (high) state.
brackets are optional.
f
XCK
f
OSC
4
-----------
RxD / TxD
XCK
RxD / TxD
XCK
UCPOL = 0
UCPOL = 1
Sample