Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 191
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 UBRR value that gives an acceptable low error can be used if possible.
19.10 Multi-processor Communication mode
Setting the Multi-processor Communication mode (MPCMn) bit in UCSRnA enables a filtering function of
incoming frames received by the USART Receiver. Frames that do not contain address information will be
ignored and not put into the receive buffer. This effectively reduces the number of incoming frames that has to
be handled by the CPU, in a system with multiple MCUs that communicate via the same serial bus. The
Transmitter is unaffected by the MPCMn setting, but has to be used differently when it is a part of a system
utilizing the Multi-processor Communication mode.
If the Receiver is set up to receive frames that contain 5 to 8 data bits, then the first stop bit indicates if the frame
contains data or address information. If the Receiver is set up for frames with nine data bits, then the ninth bit
(RXB8n) is used for identifying address and data frames. When the frame type bit (the first stop or the ninth bit)
is one, the frame contains an address. When the frame type bit is zero the frame is a data frame.
The Multi-processor Communication mode enables several slave MCUs to receive data from a master MCU.
This is done by first decoding an address frame to find out which MCU has been addressed. If a particular slave
MCU has been addressed, it will receive the following data frames as normal, while the other slave MCUs will
ignore the received frames until another address frame is received.
Table 19-2. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode (U2Xn = 0)
D
# (Data+Parity Bit)
R
slow
[%] R
fast
[%]
Maximum 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 19-3. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode (U2Xn = 1)
D
# (Data+Parity Bit) R
slow
[%] R
fast
[%]
Maximum 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