Atmel AT90S8515-8PI - Reset Sources; Power-On Reset Behavior

AT90S4414/8515
22
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 FSTRT Fuse bit in the
Flash 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 build-in start-up delay is sufficient, RESET
can be connected to V
CC
directly or via an external pull-up resistor. By
holding 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 on this.
Figure 24. MCU Start-up, RESET
Tied to V
CC
.
Figure 25. MCU Start-up, RESET
Controlled Externally
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST