ATmega32U4 Atmel Corporation, ATmega32U4 Datasheet - Page 38

• Manufacturer Part Number: ATmega32U4
• Manufacturer: Atmel Corporation

Specifications of ATmega32U4

• Flash (kbytes): 32 Kbytes
• Pin Count: 44
• Max. Operating Frequency: 16 MHz
• Cpu: 8-bit AVR
• # Of Touch Channels: 14
• Hardware Qtouch Acquisition: No
• Max I/o Pins: 26
• Ext Interrupts: 13
• Usb Transceiver: 1
• Usb Speed: Full Speed
• Usb Interface: Device
• Spi: 2
• Twi (i2c): 1
• Uart: 1
• Graphic Lcd: No
• Video Decoder: No
• Camera Interface: No
• Adc Channels: 12
• Adc Resolution (bits): 10
• Adc Speed (ksps): 15
• Analog Comparators: 1
• Resistive Touch Screen: No
• Temp. Sensor: Yes
• Crypto Engine: No
• Sram (kbytes): 3.3
• Eeprom (bytes): 1024
• Self Program Memory: YES
• Dram Memory: No
• Nand Interface: No
• Picopower: No
• Temp. Range (deg C): -40 to 85
• I/o Supply Class: 2.7 to 5.5
• Operating Voltage (vcc): 2.7 to 5.5
• Fpu: No
• Mpu / Mmu: no / no
• Timers: 4
• Output Compare Channels: 12
• Input Capture Channels: 2
• Pwm Channels: 8
• 32khz Rtc: No
• Calibrated Rc Oscillator: Yes

6.9.2
7766F–AVR–11/10
CLKPR – Clock Prescaler Register
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 – CLKPS[3..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
“0011”, giving a division factor of 8 at start up. This feature should be used if the selected clock
source has a higher frequency than the maximum frequency of the device at the present operat-
ing conditions. Note that any value can be written to the CLKPS bits regardless of the CKDIV8
Fuse setting. The Application software must ensure that a sufficient division factor is chosen if
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.
7
CLKPCE
R/W
0
6
–
R
0
5
–
R
0
4
–
R
0
3
CLKPS3
R/W
See Bit Description
2
R/W
CLKPS2
ATmega16/32U4
1
CLKPS1
R/W
0
CLKPS0
R/W
CLKPR
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