atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 35

• Manufacturer Part Number: atmega32u4-16mu
• Description: Atmega32u4 8-bit Avr Microcontroller With 32k Bytes Of Isp Flash And Usb Controller
• Manufacturer: ATMEL Corporation

Clock Selection
Register 1 – CLKSEL1
Clock Status Register
– CLKSTA
7766A–AVR–03/08
• 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 7-4 – RCCKSEL[3:0]: CKSEL for RC oscillator
Clock configuration for the RC Oscillator. After a reset, this part of the register is loaded with the
0010b value that corresponds to the RC oscillator. Modifying this value by firmware before
switching to RC oscillator is prohibited because the RC clock will not start.
• Bit 3-0 – EXCKSEL[3:0]: CKSEL for External Clock / Low Power Crystal Oscillator
Clock configuration for the External Clock / Low Power Crystal Oscillator. After a reset, if the
External Clock / Low Power Crystal Oscillator is selected by fuse bits, this part of the register is
loaded with the fuse configuration. Firmware can modify it to change the start-up time after the
clock switch.
See
Power Crystal Oscillator, Calibrated Internal RC Oscillator, and External Clock modes are
allowed.
• Bit 7-2 - Reserved bits
Bit
Read/Write
Initial Value
Bit
Read/Write
Initial Value
“Device Clocking Options Select(1)” on page 27
7
RCCKS
EL3
R/W
0
7
-
R
0
6
RCCKS
EL2
R/W
0
6
-
R
0
5
RCCKS
EL1
R/W
1
5
-
R
0
4
RCCKS
EL0
R/W
0
4
-
R
0
3
0
3
R
0
EXCKS
EL3
R/W
-
for EXCKSEL[3:0] configuration. Only Low
2
EXCKS
EL2
R/W
0
2
-
R
1
EXCKS
EL1
R/W
0
1
RCON
R
ATmega32U4
0
EXCKS
EL0
R/W
0
0
EXTON
R
CLKSEL1
CLKSTA
35