Rainbow Electronics ATmega8HVD - Manual (Page 23)

23
8052B–AVR–09/08
ATmega4HVD/8HVD
ADC. The dedicated ADC clock allows halting the CPU and I/O clocks in order to reduce noise
generated by digital circuitry. This gives more accurate ADC conversion results.
8.1.5 Watchdog Timer and Battery Protection Clock
The Watchdog Timer and Current Protection are provided with a dedicated clock domain. This
allows operation in all modes except Power-off. It also allows low power operation by utilizing
a Ultra Low Power Oscillator dedicated to this purpose.
8.2 Clock Sources
The following section describe the clock sources available in the device. The clocks are input
to the AVR clock generator, and routed to the appropriate modules.
8.3 Calibrated Fast RC Oscillator
The calibrated Fast RC Oscillator by default provides a 8.0 MHz clock to the system clock
prescaler. The frequency is nominal value at 85
° C. This clock will operate with no external
components. With an accurate time reference and by using runtime calibration, this oscillator
can be calibrated to an accuracy of ± 1% over the entire temperature range. During reset,
hardware loads the calibration byte into the FOSCCAL Register and thereby automatically cal-
ibrates the Fast RC Oscillator. At 85
° C, this calibration gives a frequency of 8 MHz ± 4%. The
oscillator can be calibrated to any frequency in the range 7.3- 8.1 MHz by changing the FOS-
CCAL register. For more information on the pre-programmed calibration value, see the section
”Reading the Signature Row from Software” on page 124 . Note that the frequency of the sys-
tem clock is given by the ”System Clock Prescaler” on page 25 .
When this Oscillator is selected, start-up times are determined by the SUT Fuses as shown in
Table 8-1 on page 23 .
Notes: 1. The device is shipped with this option selected.
2. The actual value of the added, selectable 4- 512 ms delay depends on the actual frequency
of the ”Ultra Low Power RC Oscillator” on page 24 . See Table 8-2 on page 25 and ”Electri-
cal Characteristics” on page 142
Table 8-1. Start-up times for the calibrated Fast RC Oscillator clock selection
SUT3..0
Start-up Time from
Power-save Additional Delay from Reset, Typical Values (2)
000 6 CK 14 CK + 4 ms
001 6 CK 14 CK + 8 ms
010 6 CK 14 CK + 16 ms
011 6 CK 14 CK + 32 ms
100 6 CK 14 CK + 64 ms
101 6 CK 14 CK + 128 ms
110 6 CK 14 CK + 256 ms
111 (1) 6 CK 14 CK + 512 ms