Datasheet

214
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
Figure 22-5. Start Bit Sampling
When the clock recovery logic detects a high (idle) to low (start) transition on the RxDn 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.
22.8.2 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.
Figure 22-6 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 22-6. Sampling of Data and Parity Bit
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 regis-
tered to be a logic 0. This majority voting process acts as a low pass filter for the incoming signal on the RxDn 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.
Figure 22-7 shows the sampling of the stop bit and the earliest possible beginning of the start bit of the next frame.
Figure 22-7. Stop Bit Sampling and Next Start Bit Sampling
12345678 9 10 11 12 13 14 15 16 12
STARTIDLE
00
BIT 0
3
1234 5 678120
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
12345678 9 10 11 12 13 14 15 16 1
BIT n
1234 5 6781
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
12345678 9 10 0/1 0/1 0/1
STOP 1
1234 5 6 0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A) (B) (C)