AT90S1200
Figure 13. Reset Logic
POR
Power-on Reset
Circuit
VCC
100 - 500K
Reset Circuit
RESET
Q
S
Watchdog
Timer
Time-out
On-chip
RC Oscillator
14-stage Ripple Counter
R
Q
Table 3. Reset Characteristics (VCC = 5.0V)
Symbol Parameter
Min
0.8
0.2
–
Typ
1.2
0.4
–
Max
1.6
Units
V
Power-on Reset Threshold Voltage (rising)
(1)
VPOT
Power-on Reset Threshold Voltage (falling)
0.6
V
VRST
tPOR
Pin Threshold Voltage
Power-on Reset Period
0.85 VCC
4.0
V
2.0
3.0
ms
Reset Delay Time-out Period (The Time-out
period equals 16K WDT cycles. See “Typical
Characteristics” on page 51. for typical WDT
frequency at different voltages).
tTOUT
11.0 16.0
21.0
ms
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
.
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
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
0838H–AVR–03/02
ATMEL [ ATMEL ]
