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AT90S1200

8-Bit Microcontroller with 1K bytes In-System Programmable Flash

Atmel Corporation 

Power-on Reset

0838H–AVR–03/02

AT90S1200

Figure 13. Reset Logic

VCC

Power-on Reset

Circuit

100 - 500K

RESET

Reset Circuit

POR

S

Q

Watchdog

Timer

On-chip

RC Oscillator

14-stage Ripple Counter Time-out R

Q

Table 3. Reset Characteristics (VCC = 5.0V)

Symbol Parameter

Min Typ Max Units

Power-on Reset Threshold Voltage (rising)

0.8 1.2

1.6

V

VPOT(1)

Power-on Reset Threshold Voltage (falling)

0.2 0.4

0.6

V

VRST

Pin Threshold Voltage

–

– 0.85 VCC

V

tPOR

Power-on Reset Period

2.0 3.0

4.0

ms

Reset Delay Time-out Period (The Time-out

tTOUT

period equals 16K WDT cycles. See “Typical

Characteristics” on page 51. for typical WDT

11.0 16.0

21.0

ms

frequency at different voltages).

Note: 1. The Power-on Reset will not work unless the supply voltage has been below VPOT

(falling).

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

shown in Figure 13, 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 14).

Figure 14. MCU Start-up, RESET Tied to VCC.

VCC

VPOT

RESET

VRST

TIME-OUT

tTOUT

INTERNAL

RESET

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 RESET pin low for a period after VCC has

13

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