ATMEGA325-16AJ ATMEL [ATMEL Corporation], ATMEGA325-16AJ Datasheet - Page 26

• Manufacturer Part Number: ATMEGA325-16AJ
• Description: 8-bit Microcontroller with In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

ADC Clock – clk
Clock Sources
Default Clock Source
Crystal Oscillator
26
ATmega325/3250/645/6450
ADC
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
Note:
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
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.
Device Clocking Option
External Crystal/Ceramic Resonator
External Low-frequency Crystal
Calibrated Internal RC Oscillator
External Clock
Reserved
Typ Time-out (V
1. For all fuses “1” means unprogrammed while “0” means programmed.
4.1 ms
65 ms
CC
= 5.0V)
Typ Time-out (V
(1)
4.3 ms
69 ms
CC
= 3.0V)
0011, 0001, 0101, 0100
Number of Cycles
64K (65,536)
4K (4,096)
1111 - 1000
0111 - 0110
CKSEL3..0
0010
0000
2570A–AVR–09/04