Datasheet
179
ATtiny828 [DATASHEET]
8371A–AVR–08/12
Table 61. Recommended Maximum Receiver Baud Rate Error in Normal Speed Mode
Table 62. Recommended Maximum Receiver Baud Rate Error in Double Speed Mode
The recommendations of the maximum receiver baud rate error are made under the assumption that the receiver and
transmitter divide the maximum total error equally.
There are two possible sources for the receivers baud rate error:
z The system clock of the receiver will always have some minor instability over the supply voltage range and the
temperature range
z The second source for 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
should be used, if possible
17.8.4 Start Frame Detection
The USART start frame detector can wake up the MCU when it detects a start bit. See Table 9 on page 34.
When a high-to-low transition is detected on RxDn, the internal 8 MHz oscillator is powered up and the USART clock is
enabled. After start-up the rest of the data frame can be received, provided that the baud rate is slow enough in relation
to the internal 8 MHz oscillator start-up time. Start-up time of the internal 8 MHz oscillator varies with supply voltage and
temperature.
The USART start frame detection works both in asynchronous and synchronous modes. It is enabled by writing the Start
Frame Detection Enable bit (SFDE) in “UCSRD – USART Control and Status Register D” . If the USART Start Interrupt
Enable (RXSIE) bit is set, the USART Receive Start Interrupt is generated immediately when start is detected.
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
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