Rainbow Electronics AT91CAP9S250A User Manual
Page 539
539
6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
Figure 35-12. Asynchronous Start Detection
Figure 35-13. Asynchronous Character Reception
35.6.3.4
Manchester Decoder
When the MAN field in US_MR register 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 indepen-
dent 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 addi-
tion, the polarity of the input stream is programmable with RX_MPOL field 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
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 dur-
ing one quarter of a bit time at zero, a start bit is detected. See
.. The sample pulse
rejection mechanism applies.
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