749
SAM4CP [DATASHEET]
43051E–ATPL–08/14
Figure 36-11
and
Figure 36-12
illustrate start detection and character reception when USART operates in asynchronous
mode.
Figure 36-11. Asynchronous Start Detection
Figure 36-12. Asynchronous Character Reception
36.6.3.4 Manchester Decoder
When the MAN bit in the US_MR is set to 1, the Manchester decoder is enabled. The decoder performs both preamble
and start frame delimiter detection. One input line is dedicated to Manchester encoded input data.
An optional preamble sequence can be defined, its length is user-defined and totally independent of the emitter side. Use
RX_PL in US_MAN register to configure the length of the preamble sequence. If the length is set to 0, no preamble is
detected and the function is disabled. In addition, the polarity of the input stream is programmable with RX_MPOL bit in
US_MAN register. Depending on the desired application the preamble pattern matching is to be defined via the RX_PP
field in US_MAN. See
Figure 36-8
for available preamble patterns.
Unlike preamble, the start frame delimiter is shared between Manchester Encoder and Decoder. So, if ONEBIT field is
set to 1, only a zero encoded Manchester can be detected as a valid start frame delimiter. If ONEBIT is set to 0, only
a sync pattern is detected as a valid start frame delimiter. Decoder operates by detecting transition on incoming stream.
If RXD is sampled during one quarter of a bit time to zero, a start bit is detected. See
Figure 36-13
. The sample pulse
rejection mechanism applies.
The RXIDLEV bit in the US_MAN informs the USART of the receiver line idle state value (receiver line inactive). The user
must define RXIDLEV to ensure reliable synchronization. By default, RXIDLEV is set to 1 (receiver line is at level 1 when
there is no activity).
Sampling
Clock (x16)
RXD
Start
Detection
Sampling
Baud Rate
Clock
RXD
Start
Rejection
Sampling
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
0
1
2
3
4
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
D0
Sampling
D0
D1
D2
D3
D4
D5
D6
D7
RXD
Parity
Bit
Stop
Bit
Example: 8-bit, Parity Enabled
Baud Rate
Clock
Start
Detection
16
samples
16
samples
16
samples
16
samples
16
samples
16
samples
16
samples
16
samples
16
samples
16
samples