Datasheet

24
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
9.1.4 Asynchronous Timer Clock – clk
ASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly from an external
32kHz clock crystal. The dedicated clock domain allows using this Timer/Counter as a real-time counter even when
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 as 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 “Register Summary” on
page 316.
9.3 Default Clock Source
The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source setting is therefore the 1MHz
Internal RC Oscillator with longest startup time. This default setting ensures that all users can make their desired
clock source setting using an In-System or Parallel Programmer.
9.4 Crystal Oscillator
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 9-2. Either a quartz crystal or a ceramic resonator may be used. The
CKOPT Fuse selects between two different Oscillator amplifier modes. When CKOPT is programmed, the Oscilla-
tor output will oscillate will a full rail-to-rail swing on the output. This mode is suitable when operating in a very noisy
environment or when the output from XTAL2 drives a second clock buffer. This mode has a wide frequency range.
When CKOPT is unprogrammed, the Oscillator has a smaller output swing. This reduces power consumption con-
siderably. This mode has a limited frequency range and it can not be used to drive other clock buffers.
Table 9-1. Device Clocking Options Select
(1)
Device Clocking Option CKSEL3:0
External Crystal/Ceramic Resonator 1111 - 1010
External Low-frequency Crystal 1001
External RC Oscillator 1000 - 0101
Calibrated Internal RC Oscillator 0100 - 0001
External Clock 0000
Table 9-2. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
4.1ms 4.3ms 4K (4,096)
65ms 69ms 64K (65,536)