Rainbow Electronics Atmega169L - Manual (Page 24)

24
ATmega169V/L
2514A–AVR–08/02
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 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 “ATmega169 Typical Characteristics – Preliminary
Data” on page 301.
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 2. Device Clocking Options Select (1)
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 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)