ATMEGA48V_11 ATMEL [ATMEL Corporation], ATMEGA48V_11 Datasheet - Page 174

• Manufacturer Part Number: ATMEGA48V_11
• Description: 8-bit Atmel Microcontroller with 4/8/16K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

20.3.2
2545T–AVR–05/11
Double speed operation (U2Xn)
Table 20-1
ing the UBRRn value for each mode of operation using an internally generated clock source.
Table 20-1.
Note:
Some examples of UBRRn values for some system clock frequencies are found in
page
The transfer rate can be doubled by setting the U2Xn bit in UCSRnA. Setting this bit only has
effect for the asynchronous operation. Set this bit to zero when using synchronous operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling
the transfer rate for asynchronous communication. Note however that the Receiver will in this
case only use half the number of samples (reduced from 16 to 8) for data sampling and clock
recovery, and therefore a more accurate baud rate setting and system clock are required when
this mode is used. For the Transmitter, there are no downsides.
Operating mode
Asynchronous normal mode
(U2Xn = 0)
Asynchronous double speed
mode (U2Xn = 1)
Synchronous master mode
195.
BAUD
f
UBRRn
OSC
1. The baud rate is defined to be the transfer rate in bit per second (bps)
contains equations for calculating the baud rate (in bits per second) and for calculat-
Equations for calculating baud rate register setting.
Baud rate (in bits per second, bps)
System clock frequency
Contents of the UBRRnH and UBRRnL registers, (0-4095)
BAUD
BAUD
BAUD
calculating baud rate
Equation for
=
=
=
----------------------------------------- -
16 UBRRn
-------------------------------------- -
8 UBRRn
-------------------------------------- -
2 UBRRn
(
(
(
f
f
f
OSC
OSC
OSC
+
+
+
(1)
1
1
1
)
)
)
ATmega48/88/168
UBRRn
UBRRn
UBRRn
calculating UBRRn value
Equation for
=
=
=
----------------------- - 1
16BAUD
------------------- - 1
8BAUD
------------------- - 1
2BAUD
f
f
f
OSC
OSC
OSC
Table 20-9 on
–
–
–
174