AT90USB1287-16AUR Atmel, AT90USB1287-16AUR Datasheet - Page 47
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AT90USB1287-16AUR
Manufacturer Part Number
AT90USB1287-16AUR
Description
IC MCU AVR 128K ISP USB TQFP64
Manufacturer
Atmel
Series
AVR® 90USBr
Datasheet
1.AT90USB646-MU.pdf
(461 pages)
Specifications of AT90USB1287-16AUR
Core Processor
AVR
Core Size
8-Bit
Speed
16MHz
Connectivity
EBI/EMI, I²C, SPI, UART/USART, USB, USB OTG
Peripherals
Brown-out Detect/Reset, POR, PWM, WDT
Number Of I /o
48
Program Memory Size
128KB (128K x 8)
Program Memory Type
FLASH
Eeprom Size
4K x 8
Ram Size
8K 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
64-TQFP, 64-VQFP
For Use With
ATSTK600 - DEV KIT FOR AVR/AVR32ATSTK525 - KIT STARTER FOR AT90USBAT90USBKEY2 - KIT DEMO FOR AT90USB
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Available stocks
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Manufacturer
Quantity
Price
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- Download datasheet (5Mb)
6.9
6.9.1
7593K–AVR–11/09
System Clock Prescaler
Clock Prescale Register – CLKPR
The AT90USB64/128 has a system clock prescaler, and the system clock can be divided by set-
ting the
the system clock frequency and the power consumption when the requirement for processing
power is low. This can be used with all clock source options, and it will affect the clock frequency
of the CPU and all synchronous peripherals. clk
factor as shown in
When switching between prescaler settings, the System Clock Prescaler ensures that no
glitches occurs in the clock system. It also ensures that no intermediate frequency is higher than
neither the clock frequency corresponding to the previous setting, nor the clock frequency corre-
sponding to the new setting.
The ripple counter that implements the prescaler runs at the frequency of the undivided clock,
which may be faster than the CPU's clock frequency. Hence, it is not possible to determine the
state of the prescaler - even if it were readable, and the exact time it takes to switch from one
clock division to the other cannot be exactly predicted. From the time the CLKPS values are writ-
ten, it takes between T1 + T2 and T1 + 2 * T2 before the new clock frequency is active. In this
interval, 2 active clock edges are produced. Here, T1 is the previous clock period, and T2 is the
period corresponding to the new prescaler setting.
To avoid unintentional changes of clock frequency, a special write procedure must be followed
to change the CLKPS bits:
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is
not interrupted.
• Bit 7 – CLKPCE: Clock Prescaler Change Enable
The CLKPCE bit must be written to logic one to enable change of the CLKPS bits. The CLKPCE
bit is only updated when the other bits in CLKPR are simultaneously written to zero. CLKPCE is
cleared by hardware four cycles after it is written or when CLKPS bits are written. Rewriting the
CLKPCE bit within this time-out period does neither extend the time-out period, nor clear the
CLKPCE bit.
• Bits 3..0 – CLKPS3..0: Clock Prescaler Select Bits 3 - 0
These bits define the division factor between the selected clock source and the internal system
clock. These bits can be written run-time to vary the clock frequency to suit the application
requirements. As the divider divides the master clock input to the MCU, the speed of all synchro-
nous peripherals is reduced when a division factor is used. The division factors are given in
Table
The CKDIV8 Fuse determines the initial value of the CLKPS bits. If CKDIV8 is unprogrammed,
the CLKPS bits will be reset to “0000”. If CKDIV8 is programmed, CLKPS bits are reset to
Bit
Read/Write
Initial Value
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
CLKPR to zero.
6-10.
“Clock Prescale Register – CLKPR” on page
R/W
7
CLKPCE
0
Table
R
6
–
0
6-10.
5
–
R
0
4
–
R
0
I/O
, clk
3
CLKPS3
R/W
See Bit Description
47. This feature can be used to decrease
ADC
, clk
2
CLKPS2
R/W
CPU
AT90USB64/128
, and clk
1
CLKPS1
R/W
FLASH
0
CLKPS0
R/W
are divided by a
CLKPR
47
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