Atmel ARM-Based Evaluation Kit AT91SAM9N12-EK AT91SAM9N12-EK Ficha De Dados

767

SAM9N12/SAM9CN11/SAM9CN12 [DATASHEET]

11063K–ATARM–05-Nov-13

If RXD is sampled during one quarter of a bit time to zero, a start bit is detected. See 

. The sample pulse

rejection mechanism applies.

In order to increase the compatibility the RXIDLV bit in the US_MAN register allows to inform the USART block of the Rx
line idle state value (Rx line undriven), it can be either level one (pull-up) or level zero (pull-down). By default this bit is set
to one (Rx line is at level 1 if undriven).

The receiver is activated and starts Preamble and Frame Delimiter detection, sampling the data at one quarter and then
three quarters. If a valid preamble pattern or start frame delimiter is detected, the receiver continues decoding with the
same synchronization. If the stream does not match a valid pattern or a valid start frame delimiter, the receiver
resynchronizes on the next valid edge.The minimum time threshold to estimate the bit value is three quarters of a bit
time. 

If a valid preamble (if used) followed with a valid start frame delimiter is detected, the incoming stream is decoded into
NRZ data and passed to USART for processing. 

 illustrates Manchester pattern mismatch. When incoming

data stream is passed to the USART, the receiver is also able to detect Manchester code violation. A code violation is a
lack of transition in the middle of a bit cell. In this case, MANE flag in the US_CSR is raised. It is cleared by writing the
US_CR with the RSTSTA bit to 1. See 

 for an example of Manchester error detection during data phase. 

Manchester

encoded

data

Txd

1

2

3

4

Sampling

Clock

(16 x)

Start 

Detection

Manchester

encoded

data

Txd

SFD

DATA

Preamble Length is set to 8

Preamble Mismatch

invalid pattern

Preamble Mismatch

Manchester coding error

Manchester

encoded

data

Txd

SFD

Preamble Length 

is set to 4

Elementary character bit time

Manchester

Coding Error

detected

sampling points

Preamble subpacket

and Start Frame Delimiter

were successfully

decoded

Entering USART character area