ATMEGA3250P-20AUR Atmel, ATMEGA3250P-20AUR Datasheet - Page 167

• Manufacturer Part Number: ATMEGA3250P-20AUR
• Description: MCU AVR 32K FLASH 20MHZ 100TQFP
• Manufacturer: Atmel
• Series: AVR® ATmegar

Specifications of ATMEGA3250P-20AUR

• Core Processor: AVR
• Core Size: 8-Bit
• Speed: 20MHz
• Connectivity: SPI, UART/USART, USI
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 69
• Program Memory Size: 32KB (16K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 1K x 8
• Ram Size: 2K x 8
• Voltage - Supply (vcc/vdd): 2.7 V ~ 5.5 V
• Data Converters: A/D 8x10b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 85°C
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

18.3.2
18.3.3
8023F–AVR–07/09
Double Speed Operation (U2Xn)
External Clock
Figure 18-3. Equations for Calculating Baud Rate Register Setting
Note:
Some examples of UBRRn values for some system clock frequencies are found in
(see
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.
External clocking is used by the synchronous slave modes of operation. The description in this
section refers to
External clock input from the XCK pin is sampled by a synchronization register to minimize the
chance of meta-stability. The output from the synchronization register must then pass through
an edge detector before it can be used by the Transmitter and Receiver. This process intro-
duces a two CPU clock period delay and therefore the maximum external XCK clock frequency
is limited by the following equation:
Note that f
add some margin to avoid possible loss of data due to frequency variations.
Operating Mode
Asynchronous Normal
mode (U2Xn = 0)
Asynchronous Double
Speed mode
(U2Xn = 1)
Synchronous Master
mode
page
BAUD
f
UBRRn
OSC
1. The baud rate is defined to be the transfer rate in bit per second (bps)
osc
187).
depends on the stability of the system clock source. It is therefore recommended to
Figure 18-2
Baud rate (in bits per second, bps)
System Oscillator clock frequency
Contents of the UBRRnH and UBRRnL Registers, (0-4095)
for details.
Equation for Calculating Baud
BAUD
BAUD
BAUD
=
=
=
----------------------------------------- -
16 UBRRn
Rate
-------------------------------------- -
2 UBRRn
-------------------------------------- -
8 UBRRn
f
(
XCK
(
(
(1)
f
f
OSC
f
<
OSC
OSC
f
---------- -
OSC
4
+
+
+
1
1
1
)
)
)
ATmega325P/3250P
Equation for Calculating UBRRn
UBRRn
UBRRn
UBRRn
=
Value
=
=
------------------- - 1
8BAUD
----------------------- - 1
16BAUD
------------------- - 1
2BAUD
f
OSC
f
f
OSC
OSC
Table 18-3
–
–
–
167