Rainbow Electronics ATmega32L - Manual (Page 23)

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ATmega32(L)
2503C–AVR–10/02
Asynchronous Timer Clock –
clk
ASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked
directly from an external 32 kHz clock crystal. The dedicated clock domain allows using
this Timer/Counter as a real-time counter even when the device is in sleep mode.
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 accu-
rate ADC conversion results.
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 operation 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 3. The frequency of the Watchdog
Oscillator is voltage dependent as shown in “ATmega32 Typical Characteristics – Pre-
liminary Data” on page 292.
Default Clock Source The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source
setting is therefore the Internal RC Oscillator with longest startup time. This default set-
ting ensures that all users can make their desired clock source setting using an In-
System or Parallel Programmer.
Table 2. 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 3. Number of Watchdog Oscillator Cycles
Typ Time-out (V
CC
= 5.0V) Typ Time-out (V
CC
= 3.0V) Number of Cycles
4.1 ms 4.3 ms 4K (4,096)
65 ms 69 ms 64K (65,536)