
28
2543L–AVR–08/10
ATtiny2313
128 kHz Internal
Oscillator
The 128 kHz Internal Oscillator is a low power Oscillator providing a clock of 128 kHz. The fre-
quency is nominal at 3 V and 25
°C. This clock may be selected as the system clock by
programming the CKSEL Fuses to 0110.
When this clock source is selected, start-up times are determined by the SUT Fuses as shown in
System Clock
Prescalar
The ATtiny2313 has a system clock prescaler, and the system clock can be divided by setting
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. clkI/O, clkCPU, and clkFLASH are divided by a factor as
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 corre-
sponding 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 writ-
ten, 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.
CLKPR – Clock
Prescale Register
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
Table 11. Start-up Times for the 128 kHz Internal Oscillator
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset
Recommended Usage
00
6 CK
14CK
BOD enabled
01
6 CK
14CK + 4 ms
Fast rising power
10
6 CK
14CK + 64 ms
Slowly rising power
11
Reserved
Bit
7
6
5
4
3
210
CLKPCE
–
CLKPS3
CLKPS2
CLKPS1
CLKPS0
CLKPR
Read/Write
R/W
R
R/W
Initial Value
0
See Bit Description