AT90S8515 데이터 시트보기 (PDF) - Atmel Corporation

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AT90S8515

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

Atmel Corporation 

Power-on Reset

The user can select the start-up time according to typical oscillator start-up. The number

of WDT oscillator cycles used for each time-out is shown in Table 4. The frequency of

the Watchdog Oscillator is voltage-dependent as shown in “Typical Characteristics” on

page 95.

Table 4. Number of Watchdog Oscillator Cycles

FSTRT

Programmed

Time-out at VCC = 5V

0.28 ms

Unprogrammed

16.0 ms

Number of WDT Cycles

256

16K

A Power-on Reset (POR) circuit ensures that the device is reset from power-on. As

shown in Figure 23, an internal timer clocked from the Watchdog Timer oscillator pre-

vents the MCU from starting until after a certain period after VCC has reached the Power-

on Threshold Voltage (VPOT), regardless of the VCC rise time (see Figure 24). The

FSTRT Fuse bit in the Flash can be programmed to give a shorter start-up time if a

ceramic resonator or any other fast-start oscillator is used to clock the MCU.

If the built-in start-up delay is sufficient, RESET can be connected to VCC directly or via

an external pull-up resistor. By holding the pin low for a period after VCC has been

applied, the Power-on Reset period can be extended. Refer to Figure 25 for a timing

example of this.

Figure 24. MCU Start-up, RESET Tied to VCC.

VCC

VPOT

RESET

VRST

TIME-OUT

tTOUT

INTERNAL

RESET

Figure 25. MCU Start-up, RESET Controlled Externally

VCC

VPOT

RESET

VRST

TIME-OUT

tTOUT

INTERNAL

RESET

24 AT90S8515

0841G–09/01

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