Datasheet

Figure 25-5 Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1 2
STARTIDLE
00
BIT 0
3
1 2 3 4 5 6 7 8 1 20
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
When the clock recovery logic detects a high (idle) to low (start) transition on the RxD line, the start bit
detection sequence is initiated. Let sample 1 denote the first zero-sample as shown in the figure. The
clock recovery logic then uses samples 8, 9 and 10 for Normal mode, and samples 4, 5 and 6 for Double
Speed mode (indicated with sample numbers inside boxes on the figure), to decide if a valid start bit is
received. If two or more of these three samples have logical high levels (the majority wins), the start bit is
rejected as a noise spike and the Receiver starts looking for the next high to low-transition. If however, a
valid start bit is detected, the clock recovery logic is synchronized and the data recovery can begin. The
synchronization process is repeated for each start bit.
25.8.2. Asynchronous Data Recovery
When the Receiver clock is synchronized to the start bit, the data recovery can begin. The data recovery
unit uses a state machine that has 16 states for each bit in Normal mode and eight states for each bit in
Double Speed mode. The following figure shows the sampling of the data bits and the parity bit. Each of
the samples is given a number that is equal to the state of the recovery unit.
Figure 25-6 Sampling of Data and Parity Bit
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1
BIT n
1 2 3 4 5 6 7 8 1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
The decision of the logic level of the received bit is taken by doing a majority voting of the logic value to
the three samples in the center of the received bit. The center samples are emphasized on the figure by
having the sample number inside boxes. The majority voting process is done as follows: If two or all three
samples have high levels, the received bit is registered to be a logic 1. If two or all three samples have
low levels, the received bit is registered to be a logic 0. This majority voting process acts as a low pass
filter for the incoming signal on the RxD pin. The recovery process is then repeated until a complete
frame is received. Including the first stop bit. Note that the Receiver only uses the first stop bit of a frame.
The following figure shows the sampling of the stop bit and the earliest possible beginning of the start bit
of the next frame.
Figure 25-7 Stop Bit Sampling and Next Start Bit Sampling
1 2 3 4 5 6 7 8 9 10 0/1 0/1 0/1
STOP 1
1 2 3 4 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
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