Datasheet

167
ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
20.8.3 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 Table 20-2 on page 167) 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.
D Sum of character size and parity size (D = 5 to 10 bit)
S Samples 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 20-2 on page 167 and Table 20-3 on page 167 list the maximum receiver baud rate error that can be tolerated. Note
that Normal Speed mode has higher toleration of baud rate variations.
The recommendations of the maximum receiver baud rate error was made under the assumption that the receiver and
transmitter equally divides the maximum total error.
There are two possible sources for the receivers baud rate error. The receiver’s system clock (XTAL) will always have some
minor instability over the supply voltage range and the temperature range. When using a crystal to generate the system
clock, this is rarely a problem, but for a resonator the system clock may differ more than 2% depending of the resonators
tolerance. The second source for the error is more controllable. The baud rate generator can not always do an exact division
of the system frequency to get the baud rate wanted. In this case an UBRRn value that gives an acceptable low error can be
used if possible.
Table 20-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D
# (Data+Parity Bit) R
slow
(%) R
fast
(%) Max Total Error (%)
Recommended Max Receiver
Error (%)
5 93.20 106.67 +6.67/–6.8 ±3.0
6 94.12 105.79 +5.79/–5.88 ±2.5
7 94.81 105.11 +5.11/–5.19 ±2.0
8 95.36 104.58 +4.58/–4.54 ±2.0
9 95.81 104.14 +4.14/–4.19 ±1.5
10 96.17 103.78 +3.78/–3.83 ±1.5
Table 20-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit)
R
slow
(%) R
fast
(%) Max Total Error (%)
Recommended Max Receiver
Error (%)
5 94.12 105.66 +5.66/–5.88 ±2.5
6 94.92 104.92 +4.92/–5.08 ±2.0
7 95.52 104,35 +4.35/–4.48 ±1.5
8 96.00 103.90 +3.90/–4.00 ±1.5
9 96.39 103.53 +3.53/–3.61 ±1.5
10 96.70 103.23 +3.23/–3.30 ±1.0
R
slow
D1+S
S1– DS S
F
++
---------------------------------------------
=
R
fast
D2+S
D1+S S
M
+
------------------------------------------
=