ATmega32U4 Atmel Corporation, ATmega32U4 Datasheet - Page 36

• 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

6.8.2
6.8.3
6.8.4
7766F–AVR–11/10
Switch from RC clock to external clock
CLKSEL0 – Clock Selection Register 0
CLKSEL1 – Clock Selection Register 1
• Bit 7-6 – RCSUT[1:0]: SUT for RC oscillator
These 2 bits are the SUT value for the RC Oscillator. If the RC oscillator is selected by fuse bits,
the SUT fuse are copied into these bits. A firmware change will not have any effect because this
additional start-up time is only used after a reset and not after a clock switch.
• Bit 5-4 – EXSUT[1:0]: SUT for External Clock/ Low Power Crystal Oscillator
These 2 bits are the SUT value for the External Clock / Low Power Crystal Oscillator. If the
External Clock / Low Power Crystal Oscillator is selected by fuse bits, the SUT fuses are copied
into these bits. The firmware can modify these bits by writing a new value. This value will be
used at the next start of the External Clock / Low Power Crystal Oscillator.
• Bit 3 – RCE: Enable RC Oscillator
The RCE bit must be written to logic one to enable the RC Oscillator. The RCE bit must be writ-
ten to logic zero to disable the RC Oscillator.
• Bit 2 – EXTE: Enable External Clock / Low Power Crystal Oscillator
The OSCE bit must be written to logic one to enable External Clock / Low Power Crystal Oscilla-
tor. The OSCE bit must be written to logic zero to disable the External Clock / Low Power Crystal
Oscillator.
• Bit 0 – CLKS: Clock Selector
The CLKS bit must be written to logic one to select the External Clock / Low Power Crystal Oscil-
lator as CPU clock. The CLKS bit must be written to logic zero to select the RC Oscillator as
CPU clock. After a reset, the CLKS bit is set by hardware if the External Clock / Low Power Crys-
tal Oscillator is selected by the fuse bits configuration.
The firmware has to check if the clock is correctly started before selected it.
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
if (Usb_wake_up_detected())
{
}
Usb_ack_wake_up();
Enable_external_clock();
while (!External_clock_ready()); // while (CLKSTA.EXTON != 1);
Select_external_clock();
Enable_pll();
Disable_RC_clock();
while (!Pll_ready());
Usb_unfreeze_clock();
7
RCSUT1
R/W
0
7
RCCKS
EL3
R/W
0
6
RCCKS
EL2
R/W
0
6
RCSUT0
R/W
0
5
RCCKS
EL1
R/W
1
5
EXSUT1
R/W
0
4
RCCKS
EL0
0
R/W
4
EXSUT0
R/W
0
3
EXCKS
EL3
R/W
0
// if (UDINT.WAKEUPI == 1)
// UDINT.WAKEUPI = 0;
// CKSEL0.EXTE = 1;
// CLKSEL0.CLKS = 1;
// PLLCSR.PLLE = 1;
// CLKSEL0.RCE = 0;
// while (PLLCSR.PLOCK != 1);
// USBCON.FRZCLK = 0;
3
RCE
R/W
See Bit Description
2
EXCKS
EL2
R/W
0
2
EXTE
R/W
0
1
EXCKS
EL1
R/W
ATmega16/32U4
-
1
R
0
EXCKS
EL0
R/W
0
0
CLKS
R/W
CLKSEL1
CLKSEL0
36