Datasheet

35
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
Figure 6-5. Example of Clock Switching with Wake-up from Device
6.8 Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be
programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock
also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is
programmed. Any clock source, including the internal RC Oscillator, can be selected when the clock is output on
CLKO.
If the System Clock Prescaler is used, it is the divided system clock that is output.
6.8.1 System Clock Prescaler
The AVR USB has a system clock prescaler, and the system clock can be divided by setting the “CLKPR –
Clock Prescaler Register” on page 39. This feature can be used to decrease 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
I/O
, clk
ADC
,
clk
CPU
, and clk
FLASH
are divided by a factor as shown in Table 6-10 on page 40.
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 corresponding 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 written, it takes between T1 + T2 and T1 + 2 * T2 before the
new clock frequency is active. In this interval, two 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:
1. Write the Clock Prescaler Change Enable (CLKPCE) bit to one and all other bits in CLKPR to zero.
2. Within four cycles, write the desired value to CLKPS while writing a zero to CLKPCE.
Interrupts must be disabled when changing prescaler setting to make sure the write procedure is not interrupted.
USB
CPU Clock
Exte rnal
Oscillator
RC oscillator
Ext RC Ext
non-Idle Idle
(Suspend)
non-Idle
3m
s
upstream-resume
2
2 Upstream Resume from device
w atchdog wake-up
from power-down