Datasheet

 2017 Microchip Technology Inc. DS60001516A-page 457
SAM9G20
Figure 31-14: Asynchronous Start Bit Detection
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 re-synchronizes 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. Figure 31-15 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, MANERR flag in US_CSR is raised. It is cleared by writing the Control Register (US_CR) with the RSTSTA bit at 1. See
Figure 31-16 for an example of Manchester error detection during data phase.
Figure 31-15: Preamble Pattern Mismatch
Figure 31-16: Manchester Error Flag
When the start frame delimiter is a sync pattern (ONEBIT field at 0), both command and data delimiter are supported. If a valid sync is
detected, the received character is written as RXCHR field in the US_RHR and the RXSYNH is updated. RXCHR is set to 1 when the
received character is a command, and it is set to 0 if the received character is a data. This mechanism alleviates and simplifies the direct
memory access as the character contains its own sync field in the same register.
As the decoder is setup to be used in unipolar mode, the first bit of the frame has to be a zero-to-one transition.
Manchester
encoded
data
Txd
123 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