AT90S/LS4434 and AT90S/LS8535
21
Note:
1. The Power-on Reset will not work unless the supply voltage has been below V
POT
(falling).
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
clocked from the Watchdog Timer oscillator prevents the MCU from starting until after a certain period after V
CC
has
reached the Power-on Threshold voltage (V
POT
), regardless of the V
CC
rise time (see Figure 24).
The user can select the start-up time according to typical oscillator start-up time. The number of WDT oscillator cycles is
shown in Table 5. The frequency of the Watchdog oscillator is voltage-dependent as shown in “Typical Characteristics” on
page 100.
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 hold-
ing the pin low for a period after V
CC
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 V
CC
.
Table 4.
Reset Characteristics (V
CC
= 5.0V)
Symbol
Parameter
Min
Typ
Max
Units
V
POT
(1)
Power-on Reset Threshold (rising)
1.0
1.4
1.8
V
Power-on Reset Threshold (falling)
0.4
0.6
0.8
V
V
RST
RESET Pin Threshold Voltage
0.6 V
CC
V
t
TOUT
Reset Delay Time-out Period
FSTRT Unprogrammed
11.0
16.0
21.0
ms
t
TOUT
Reset Delay Time-out Period
FSTRT Programmed
1.0
1.1
1.2
ms
Table 5.
Number of Watchdog Oscillator Cycles
FSTRT
Time-out at V
CC
= 5V
Number of WDT Cycles
Programmed
1.1 ms
1K
Unprogrammed
16.0 ms
16K
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
