Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 190
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 (FEn) Flag 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 Figure 19-7
on page 189. 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.
19.9.3 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 Table 19-2 on page 191) base
frequency, 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.
DSum of character size and parity size (D = 5 to 10 bit).
SSamples per bit. S = 16 for Normal Speed mode and S = 8 for Double Speed
mode.
S
F
First sample number used for majority voting. S
F
= 8 for normal speed and S
F
= 4
for Double Speed mode.
S
M
Middle sample number used for majority voting. S
M
= 9 for normal speed and
S
M
= 5 for Double Speed mode.
R
slow
is the ratio of the slowest incoming data rate that can be accepted in relation to the
receiver baud rate. R
fast
is the ratio of the fastest incoming data rate that can be
accepted in relation to the receiver baud rate.
Table 19-2 on page 191 and Table 19-3 on page 191 list the maximum receiver baud rate error that can be
tolerated. Note that Normal Speed mode has higher toleration of baud rate variations.
R
slow
D 1+S
S 1– DS S
F
++
-------------------------------------------=
R
fast
D 2+S
D 1+SS
M
+
-----------------------------------=