AT90PWM2 datasheet(209/365 Pages) ATMEL

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209

AT90PWM2/3

4317B–AVR–02/05

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.

Multi-processor

Communication Mode

This mode is available only in USART mode, not in EUSART.

Setting the Multi-processor Communication mode (MPCM) bit in UCSRA enables a fil-

tering 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 Trans-

mitter is unaffected by the MPCM setting, but has to be used differently when it is a part

of a system utilizing the Multi-processor Communication mode.

MPCM Protocol

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

Table 76. Recommended Maximum Receiver Baud Rate Error for Normal Speed Mode

(U2X = 0)

# (Data+Parity Bit)

Rslow (%)

Rfast (%)

Max Total Error (%)

Recommended Max

Receiver Error (%)

93.20

106.67

+6.67/-6.8

± 3.0

94.12

105.79

+5.79/-5.88

± 2.5

94.81

105.11

+5.11/-5.19

± 2.0

95.36

104.58

+4.58/-4.54

± 2.0

95.81

104.14

+4.14/-4.19

± 1.5

96.17

103.78

+3.78/-3.83

± 1.5

Table 77. Recommended Maximum Receiver Baud Rate Error for Double Speed Mode

(U2X = 1)

# (Data+Parity Bit)

slow (%)

fast (%)

Max Total Error (%)

Recommended Max

Receiver Error (%)

94.12

105.66

+5.66/-5.88

± 2.5

94.92

104.92

+4.92/-5.08

± 2.0

95.52

104,35

+4.35/-4.48

± 1.5

96.00

103.90

+3.90/-4.00

± 1.5

96.39

103.53

+3.53/-3.61

± 1.5

96.70

103.23

+3.23/-3.30

± 1.0

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