586
32072H–AVR32–10/2012
AT32UC3A3
The Manchester endec uses the same Start Frame Delimiter Selector (MR.ONEBIT) for both
encoder and decoder. If ONEBIT is one, only a Manchester encoded zero will be accepted as a
valid start frame delimiter. If ONEBIT is zero, a data or command sync pattern will be expected.
The Received Sync bit in the Receive Holding Register (RHR.RXSYNH) will be zero if the last
character received is a data sync, and a one if it is a command sync.
Figure 25-48. Preamble Pattern Mismatch
The receiver samples the RXD line in continuos bit period quarters, making the smallest time
frame in which to assume a bit value three quarters. A start bit is assumed if RXD is zero during
Figure 25-49. Asynchronous Start Bit Detection
If a valid preamble pattern or start frame delimiter is detected, the receiver continues decoding
with the same synchronization. If a non-valid preamble pattern or a start frame delimiter is
detected, the receiver re-synchronizes at the next valid edge. When a valid start sequence has
been detected, the decoded data is passed to the USART and the user will be notified of any
incoming Manchester encoding violations by the Manchester Error bit (CSR.MANERR). An inter-
rupt request is generated if one of the Manchester Error bits in the Interrupt Mask Register
(IMR.MANE or IMR.MANEA) is set. CSR.MANERR is cleared by writing a one to the Reset Sta-
tus bits in the Control Register (CR.RSTSTA). A violation occurs when there is no transition in
the middle of a bit period. See
Figure 25-50 for an illustration of a violation causing the Man-
chester Error bit to be set.
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
1234
Sampling
Clock
(16 x)
Start
Detection