Rainbow Electronics AT90S2313 - Manual (Page 20)

20
AT90S2313
0839I–AVR–06/02
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 is clocked from the Watchdog Timer. This timer
prevents the MCU from starting until after a certain period after V
CC
has reached the
Power-on Threshold voltage (V
POT
) (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 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
been applied, the Power-on Reset period can be extended. Refer to Figure 25 for a tim-
ing example of this.
Figure 24. MCU Start-up, RESET Tied to V
CC
.
Figure 25. MCU Start-up, RESET Controlled Externally
Table 4. Number of Watchdog Oscillator Cycles
FSTRT Time-out at V
CC
= 5V Number of WDT Cycles
Programmed 0.28 ms 256
Unprogrammed 16.0 ms 16K
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST