atmega32u4-16mu ATMEL Corporation, atmega32u4-16mu Datasheet - Page 34

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

Clock switch
Algorithm
Switch from external
clock to RC clock
Switch from RC clock to
external clock
Clock Selection
Register 0 – CLKSEL0
34
USB
CPU Clock
External
Oscillator
RC oscillator
ATmega32U4
non-Idle
Figure 2. Example of clock switching with wake-up from Device
Bit
Read/Write
Initial Value
if (Usb_suspend_detected())
{
}
if (Usb_wake_up_detected())
{
}
Ext
Usb_ack_suspend();
Usb_freeze_clock();
Disable_pll();
Enable_RC_clock();
while (!RC_clock_ready());
Select_RC_clock();
Disable_external_clock();
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();
3m
Idle
s
7
RCSUT1
R/W
0
6
RCSUT0
R/W
0
5
EXSUT1
R/W
0
w atchdog wake-up
from power-down
4
EXSUT0
R/W
0
(Suspend)
2 Upstream Resume from device
RC
// if (UDINT.SUSPI == 1)
// UDINT.SUSPI = 0;
// USBCON.FRZCLK = 1;
// PLLCSR.PLLE = 0;
// CLKSEL0.RCE = 1;
// while (CLKSTA.RCON != 1);
// CLKSEL0.CLKS = 0;
// CLKSEL0.EXTE = 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
R/W
EXTE
1
-
R
upstream-resume
0
CLKS
R/W
2
7766A–AVR–03/08
CLKSEL0
non-Idle
Ext