3 asynchronous operational range, Figure 23-6 on, Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
Page 353
353
8266A-MCU Wireless-12/09
ATmega128RFA1
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 23-6. Sampling of Data and Parity Bit
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9
10
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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 centre of the received bit. The centre
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 logic 1. If
two or all three samples have low levels, the received bit is registered to be 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.
shows the sampling of the stop bit and the earliest possible
beginning of the start bit of the next frame.
Figure 23-7. Stop Bit Sampling and Next Start Bit Sampling
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10
0/1
0/1
0/1
STOP 1
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2
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6
0/1
RxD
Sample
(U2X = 0)
Sample
(U2X = 1)
(A)
(B)
(C)
The same majority voting is done to the stop bit as done for the other bits in the frame.
If the stop bit is registered to have a logic 0 value, the Frame Error Flag (FEn) will be
set.
A new high to low transition indicating the start bit of a new frame can come right after
the last of the bits used for majority voting. For normal speed mode, the first low level
sample can be at point marked (A) in
. For double speed mode the
first low level must be delayed to (B). (C) marks a stop bit of full length. The early start
bit detection influences the operational range of the receiver.
23.8.3 Asynchronous Operational Range
The operational range of the receiver is dependent on the mismatch between the
received bit rate and the internally generated baud rate. If the transmitter is sending
frames at too fast or too slow bit rates, or the internally generated baud rate of the
receiver does not have a similar (see
page 354) base frequency, the
receiver will not be able to synchronize the frames to the start bit.