
13
AT90S1200
0838H–AVR–03/02
Figure 13. Reset Logic
Note:
1. The Power-on Reset will not work unless the supply voltage has been below VPOT
(falling).
Power-on Reset
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.
If the built-in start-up delay is sufficient, RESET can be connected to V
CC directly or via
an external pull-up resistor. By holding the RESET pin low for a period after V
CC has
Table 3. Reset Characteristics (V
CC = 5.0V)
Symbol
Parameter
Min
Typ
Max
Units
VPOT
Power-on Reset Threshold Voltage (rising)
0.8
1.2
1.6
V
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
tTOUT
Reset Delay Time-out Period (The Time-out
frequency at different voltages).
11.0
16.0
21.0
ms
VCC
Power-on Reset
Circuit
RESET
POR
100 - 500K
Reset Circuit
Watchdog
Timer
On-chip
RC Oscillator
Counter
Reset
14-stage Ripple Counter
Time-out
S
Q
R
Q
Inter
nal
Reset
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST