at91sam9g20-cu ATMEL Corporation, at91sam9g20-cu Datasheet - Page 473

• Manufacturer Part Number: at91sam9g20-cu
• Description: At91 Arm Thumb Microcontrollers
• Manufacturer: ATMEL Corporation

6384B–ATARM–15-Dec-08
Figure 32-14. 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.
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 US_CSR register is raised. It is cleared by
writing the Control Register (US_CR) with the RSTSTA bit at 1. See
ple of Manchester error detection during data phase.
Figure 32-15. Preamble Pattern Mismatch
Figure 32-16. Manchester Error Flag
Manchester
encoded
Manchester
Manchester
Sampling
encoded
encoded
data
(16 x)
Clock
data
data
sampling points
Txd
Txd
Txd
1
Manchester coding error
2
Preamble Length
3
Preamble Mismatch
4
is set to 4
Preamble Length is set to 8
Detection
Start
and Start Frame Delimiter
AT91SAM9G20 Preliminary
Preamble subpacket
were successfully
Preamble Mismatch
SFD
decoded
invalid pattern
Entering USART character area
Elementary character bit time
Figure 32-16
Figure 32-15
SFD
Coding Error
Manchester
for an exam-
detected
DATA
illustrates
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