Datasheet

178
ATtiny828 [DATASHEET]
8371A–AVR–08/12
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 one. If two, or all three samples have low levels, the received bit is registered to be a logic zero.
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.
Figure 76 shows the sampling of the stop bit and the earliest possible beginning of the start bit of the next frame.
Figure 76. Stop Bit Sampling and Next Start Bit Sampling
The stop bit is subject to the same majority voting as the other bits in the frame. If the stop bit is registered to have a logic
low value, the Frame Error flag (FE) 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. In normal speed mode, the first low level sample can be at point marked (A) in Figure 76. In double speed mode
the first low level must be delayed to (B). Point (C) marks the full length of a stop bit.
The early start bit detection influences the operational range of the receiver.
17.8.3 Asynchronous Operational Range
The operational range of the receiver depends 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 base frequency (see Table 61 on page 179), the receiver will not be able to
synchronize the frames to the start bit.
The following equations can be used to calculate the ratio of the incoming data rate and internal receiver baud rate.
... where:
D Sum of character size and parity size (D = 5 to 10 bit)
S Samples per bit, 16 for normal speed mode, or 8 for double speed mode.
S
F
First sample number used for majority voting, 8 (normal speed), or 4 (double)
S
M
Middle sample number for majority voting, 9 (normal speed), or 5 (double speed)
R
slow
The ratio of the slowest incoming data rate that can be accepted with respect to
the receiver baud rate.
R
fast
The ratio of the fastest incoming data rate that can be accepted with respect to
the receiver baud rate.
Table 61 on page 179 and Table 62 on page 179 list the maximum receiver baud rate error that can be tolerated. Note
that normal speed mode has higher toleration of baud rate variations.
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)
R
slow
D 1+()S
S 1– DS⋅ S
F
++
------------------------------------------
=
R
fast
D 2+()S
D 1+()SS
M
+
-----------------------------------
=