ATMEGA8515-16JU(2010) View Datasheet(PDF) - Atmel Corporation

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Description

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ATMEGA8515-16JU
(Rev.:2010)

8-bit Microcontroller with 8K Bytes In-System Programmable Flash

Atmel Corporation 

ATmega8515(L)

Flash Clock – clkFLASH

Clock Sources

Default Clock Source

Crystal Oscillator

2512K–AVR–01/10

The Flash clock controls operation of the Flash interface. The Flash clock is usually

active simultaneously with the CPU clock.

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.

Table 5. Device Clocking Options Select(1)

Device Clocking Option

External Crystal/Ceramic Resonator

CKSEL3..0

1111 - 1010

External Low-frequency Crystal

1001

External RC Oscillator

Calibrated Internal RC Oscillator

External Clock

1000 - 0101

0100 - 0001

0000

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 6. The frequency of the Watchdog Oscil-

lator is voltage dependent as shown in “ATmega8515 Typical Characteristics” on page

207.

Table 6. Number of Watchdog Oscillator Cycles

Typ Time-out (VCC = 5.0V)

4.1 ms

Typ Time-out (VCC = 3.0V)

4.3 ms

65 ms

69 ms

Number of Cycles

4K (4,096)

64K 65,536)

The device is shipped with CKSEL = “0001” and SUT = “10”. The default clock source

setting is therefore the Internal RC Oscillator with longest start-up time. This default set-

ting ensures that all users can make their desired clock source setting using an In-

System or Parallel Programming.

XTAL1 and XTAL2 are input and output, respectively, of an inverting amplifier which can

be configured for use as an On-chip Oscillator, as shown in Figure 19. Either a quartz

crystal or a ceramic resonator may be used. The CKOPT Fuse selects between two dif-

ferent Oscillator amplifier modes. When CKOPT is programmed, the Oscillator output

will oscillate will a full rail-to-rail swing on the output. This mode is suitable when operat-

ing in a very noisy environment or when the output from XTAL2 drives a second clock

buffer. This mode has a wide frequency range. When CKOPT is unprogrammed, the

Oscillator has a smaller output swing. This reduces power consumption considerably.

This mode has a limited frequency range and it can not be used to drive other clock

buffers.

For resonators, the maximum frequency is 8 MHz with CKOPT unprogrammed and

16 MHz with CKOPT programmed. C1 and C2 should always be equal for both crystals

and resonators. The optimal value of the capacitors depends on the crystal or resonator

35

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