Rainbow Electronics AT90S8515 - Manual (Page 24)

24
AT90S8515
0841G–09/01
The user can select the start-up time according to typical oscillator start-up. The number
of WDT oscillator cycles used for each time-out is shown in Table 4. The frequency of
the Watchdog Oscillator is voltage-dependent as shown in “Typical Characteristics” on
page 95.
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 pre-
vents 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 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 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
.
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