ATMEGA325 Datasheet, PDF (26/347 Pages) ATMEL Corporation – 8-bit Microcontroller with In-System Programmable Flash

ADC Clock – clk_ADC
Clock Sources
Default Clock Source
Crystal Oscillator
The ADC is provided with a dedicated clock domain. This allows halting the CPU and
I/O clocks in order to reduce noise generated by digital circuitry. This gives more accu-
rate ADC conversion results.
The device has the following clock source options, selectable by Flash Fuse bits as
shown below. The clock from the selected source is input to the AVR clock generator,
and routed to the appropriate modules.
Table 3. Device Clocking Options Select⁽¹⁾
Device Clocking Option
CKSEL3..0
External Crystal/Ceramic Resonator
1111 - 1000
External Low-frequency Crystal
0111 - 0110
Calibrated Internal RC Oscillator
0010
External Clock
0000
Reserved
0011, 0001, 0101, 0100
Note: 1. For all fuses “1” means unprogrammed while “0” means programmed.
The various choices for each clocking option is given in the following sections. When the
CPU wakes up from Power-down or Power-save, the selected clock source is used to
time the start-up, ensuring stable Oscillator operation before instruction execution starts.
When the CPU starts from reset, there is an additional delay allowing the power to reach
a stable level before commencing normal operation. The Watchdog Oscillator is used
for timing this real-time part of the start-up time. The number of WDT Oscillator cycles
used for each time-out is shown in Table 4. The frequency of the Watchdog Oscillator is
voltage dependent as shown in “ATmega325/3250/645/6450 Typical Characteristics –
Preliminary Data” on page 292.
Table 4. Number of Watchdog Oscillator Cycles
Typ Time-out (V_CC = 5.0V)
4.1 ms
Typ Time-out (V_CC = 3.0V)
4.3 ms
65 ms
69 ms
Number of Cycles
4K (4,096)
64K (65,536)
The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed.
The default clock source setting is the Internal RC Oscillator with longest start-up time
and an initial system clock prescaling of 8. This default setting ensures that all users can
make their desired clock source setting using an In-System or Parallel programmer.
XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can
be configured for use as an On-chip Oscillator, as shown in Figure 13. Either a quartz
crystal or a ceramic resonator may be used.
C1 and C2 should always be equal for both crystals and resonators. The optimal value
of the capacitors depends on the crystal or resonator in use, the amount of stray capac-
itance, and the electromagnetic noise of the environment. Some initial guidelines for
choosing capacitors for use with crystals are given in Table 5. For ceramic resonators,
the capacitor values given by the manufacturer should be used.
26 ATmega325/3250/645/6450
2570A–AVR–09/04