32
ATmega8515(L)
2512A–AVR–04/02
Flash Clock – clk
FLASH
The Flash clock controls operation of the Flash interface. The Flash clock is usually
active simultaneously with the CPUclock.
Clock Sources The device has the following clock sourceoptions, selectable by Flash Fuse bits as
shown below.The clock from the selectedsourceis input to the AVR clock generator,
androuted to theappropriate modules.
Note: 1. For all fuses “1” means unprogrammedwhile“0”meansprogrammed.
Thevariouschoicesfor each clocking option is giveninthe following sections. When the
CPUwakes upfrom Power-downor Power-save, the selectedclock sourceis used to
time the start-up, ensuring stable Oscillator operation beforeinstruction execution starts.
When the CPUstartsfrom Reset, thereis as an additionaldelay allowing the power to
reach a stable levelbefore commencing normal operation. The Watchdog Oscillator is
usedfor timing thisreal-time partof the start-up time. Thenumber ofWDT Oscillator
cycles usedfor each time-out isshowninTable 6.The frequency of the Watchdog Oscil-
lator is voltage dependent asshown in “ATmega8515 TypicalCharacteristics –
Preliminary Data” on page 204. The deviceisshippedwith CKSEL = “0001” and
SUT = “10” (1 MHz Internal RCOscillator, slowly rising power).
Crystal Oscillator XTAL1andXTAL2areinput and output,respectively, of an inverting amplifierwhich can
be configuredfor useas an On-chipOscillator, asshowninFigure18.Either a quartz
crystal or a ceramicresonator maybeused.The CKOPT Fuse selectsbetween two dif-
ferent oscillator amplifier modes. When CKOPT isprogrammed, the Oscillator output
will oscillate will a full rail-to-railswingontheoutput. This modeissuitable when operat-
inginavery noisy environment orwhen theoutput from XTAL2 drives a secondclock
buffer.This mode has a wide frequency range. When CKOPT is unprogrammed, the
Oscillatorhas a smaller output swing. Thisreducespowerconsumption considerably.
This mode has a limitedfrequency range and it can not beused to drive otherclock
buffers.
Forresonators, themaximum frequency is 8 MHz withCKOPT unprogrammed and
16MHzwithCKOPT programmed. C1andC2 should always beequalforbothcrystals
andresonators.Theoptimal value of the capacitors depends on the crystal orresonator
in use, theamountofstraycapacitance, and theelectromagnetic noiseof theenviron-
ment. Some initial guidelinesforchoosing capacitors for use withcrystals aregivenin
Table 7. Forceramicresonators, the capacitor values given by the manufacturershould
beused.
Table 5. Device Clocking OptionsSelect
(1)
Device Clocking Option CKSEL3..0
ExternalCrystal/Ceramic Resonator 1111 - 1010
ExternalLow-frequency Crystal 1001
External RCOscillator 1000 - 0101
CalibratedInternal RCOscillator 0100 - 0001
ExternalClock 0000
Table 6. Number ofWatchdog 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)
65ms69ms64K65,536)
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