Rainbow-electronics AT90S2313 Manuale Utente Scaricare il pdf (Pagina 20)

AT90S2313

0839I–AVR–06/02

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

shown in Figure 23, an internal timer is clocked from the Watchdog Timer. This timer

prevents the MCU from starting until after a certain period after V

has reached the

Power-on Threshold voltage (V

POT

)(seeFigure24).TheFSTRTFusebitintheFlash

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 V

directly or via

an external pull-up resistor. By holding the RESET

pin low for a period after V

has

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

ing example of this.

Figure 24. MCU Start-up, RESET

Tied to V

Figure 25. MCU Start-up, RESET

Controlled Externally

Table 4. Number of Watchdog Oscillator Cycles

FSTRT Time-out at V

= 5V Number of WDT Cycles

Programmed 0.28 ms 256

Unprogrammed 16.0 ms 16K

VCC

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

VCC

RESET

TIME-OUT

INTERNAL

RESET

TOUT

POT

RST

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 91 92

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