Datasheet

ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]

8284E–AVR–02/2013

9.1.4 Asynchronous Timer Clock – clk

ASY

The Asynchronous Timer clock allows the Asynchronous Timer/Counter and the LCD controller to be clocked

directly from an external clock or an external 32kHz clock XTAL. The dedicated clock domain allows using this

Timer/Counter as a real-time counter even when the device is in sleep mode. It also allows the LCD controller out-

put to continue while the rest of the device is in sleep mode.

9.1.5 ADC Clock – clk

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 accurate ADC conversion results.

9.2 Clock sources

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.

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 oper-

ation 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 9-

2. The frequency of the Watchdog Oscillator is voltage dependent as shown in ”Typical Characteristics – TA = -

40°C to 85°C” on page 344.

9.3 Default clock source

The device is shipped with CKSEL = “0010”, SUT = “10”, and CKDIV8 programmed. The default clock source set-

ting 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.

9.4 Calibrated internal RC oscillator

By default, the Internal RC Oscillator provides an approximate 8MHz clock. Though voltage and temperature

dependent, this clock can be very accurately calibrated by the user. The device is shipped with the CKDIV8 Fuse

programmed. See ”System Clock Prescaler” on page 35 for more details.

Table 9-1. Device clocking options select

(1)

Device clocking option CKSEL3..0

External XTAL/ceramic resonator 1111 - 1000

External low-frequency XTAL 0111 - 0110

Calibrated internal RC oscillator 0010

External clock 0000

Reserved 0011, 0001, 0101, 0100

Table 9-2. Number of watchdog oscillator cycles.

Typ time-out (V

= 5.0V) Typ time-out (V

= 3.0V) Number of cycles

4.1ms 4.3ms 4K (4,096)

65ms 69ms 64K (65,536)