147
ATmega8515(L)
2512A–AVR–04/02
The recommendations of themaximum receiverbaudrate errorwas madeunder the
assumption that the receiver and transmitter equally divides themaximum total error.
Therearetwo possible sourcesfor the receiver’sbaudrate error.The receiver’s system
clock (XTAL) will always have some minor instability over the supply voltage range and
thetemperature range. When using a crystal to generate the system clock, this israrely
a problem,but for a resonator the system clock maydiffer morethan 2%depending of
the resonators tolerance. The secondsource for theerror is more controllable. The baud
rate generatorcan not always doanexact division of the system frequency to get the
baudrate wanted. InthiscaseanUBRR value that gives an acceptable low errorcan be
used ifpossible.
Multi-processor
Communication Mode
Setting the Multi-processorCommunication mode (MPCM) bit in UCSRA enables a fil-
tering function of incoming framesreceivedbythe USART Receiver. Frames that do not
contain address information will be ignored and not put into the receive buffer.This
effectively reduces thenumber of incoming frames that has to be handledbythe CPU,
in a system with multiple MCUs that communicate via the same serialbus.TheTrans-
mitter is unaffectedbythe MPCM setting,but has to beuseddifferently when it is a part
of a system utilizing the Multi-processorCommunication mode.
If theReceiver isset up to receive frames that contain 5 to 8 data bits, then the first stop
bit indicates if the frame containsdata or address information. If the receiver isset upfor
frameswith nine data bits, then theninthbit (RXB8) is usedfor identifying address and
data frames. When the frame type bit (the first stop or theninthbit) is one, the frame
contains an address. When the frame type bit iszerothe frame is a data frame.
Table 61. RecommendedMaximum ReceiverBaud Rate ErrorforNormalSpeedMode
(U2X=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
694.12 105.79 +5.79/-5.88 ± 2.5
794.81 105.11 +5.11/-5.19±2.0
8 95.36 104.58 +4.58/-4.54 ± 2.0
995.81 104.14 +4.14/-4.19±1.5
10 96.17 103.78 +3.78/-3.83±1.5
Table 62. RecommendedMaximum ReceiverBaud Rate ErrorforDouble SpeedMode
(U2X=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
694.92104.92 +4.92/-5.08 ± 2.0
795.52 104.35 +4.32/-4.48 ± 1.5
8 96.00 103.90 +3.90/-4.00 ± 1.5
996.39 103.53+3.53/-3.61 ± 1.5
10 96.70103.23+3.23/-3.30 ± 1.0
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