Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Fiche De Données
Codes de produits
MEGA328P-XMINI
174
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
contains equations for calculating the baud rate (in bits per second) and for calculating the UBRRn
value for each mode of operation using an internally generated clock source.
Note:
1.
The baud rate is defined to be the transfer rate in bit per second (bps)
BAUD
Baud rate (in bits per second, bps)
f
OSC
System Oscillator clock frequency
UBRRn
Contents of the UBRRnH and UBRRnL Registers, (0-4095)
Some examples of UBRRn values for some system clock frequencies are found in
).
20.3.2 Double Speed Operation (U2Xn)
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.
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.
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.
20.3.3 External Clock
External clocking is used by the synchronous slave modes of operation. The description in this section refers to
for details.
Table 20-1.
Equations for Calculating Baud Rate Register Setting
Operating Mode
Equation for Calculating Baud
Rate
Equation for Calculating
UBRRn Value
Asynchronous Normal mode
(U2Xn = 0)
(U2Xn = 0)
Asynchronous Double Speed
mode (U2Xn = 1)
mode (U2Xn = 1)
Synchronous Master mode
BAUD
f
OSC
16 UBRRn 1
+
------------------------------------------
=
UBRRn
f
OSC
16BAUD
------------------------
1
–
=
BAUD
f
OSC
8 UBRRn 1
+
---------------------------------------
=
UBRRn
f
OSC
8BAUD
--------------------
1
–
=
BAUD
f
OSC
2 UBRRn 1
+
---------------------------------------
=
UBRRn
f
OSC
2BAUD
--------------------
1
–
=