38
7679H–CAN–08/08
AT90CAN32/64/128
5.1.4
Asynchronous Timer Clock – clk
ASY
The Asynchronous Timer clock allows the Asynchronous Timer/Counter to be clocked directly
from an external clock or 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.
5.1.5
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.
5.2
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 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 starting
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
. The
frequency of the Watchdog Oscillator is voltage dependent as shown in
Typical Characteristics” on page 384
5.3
Default Clock Source
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.
Table 5-1.
Device Clocking Options Select
Device Clocking Option
CKSEL3..0
External Crystal/Ceramic Resonator
1111 - 1000
External Low-frequency Crystal
0111 - 0100
Calibrated Internal RC Oscillator
0010
External Clock
0000
Reserved
0011, 0001
Table 5-2.
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)
