Rainbow Electronics AT90S2313 - Manual (Page 19)

19
AT90S2313
0839I–AVR–06/02
Reset Sources The AT90S2313 has three sources of reset:
• Power-on Reset. The MCU is reset when the supply voltage is below the Power-on
Reset threshold (V
POT
).
• External Reset. The MCU is reset when a low level is present on the RESET pin for
more than 50 ns.
• Watchdog Reset. The MCU is reset when the Watchdog Timer period expires and
the Watchdog is enabled.
During Reset, all I/O Registers are then set to their initial values, and the program starts
execution from address $000. The instruction placed in address $000 must be an RJMP
(Relative Jump) instruction to the reset handling routine. If the program never enables
an interrupt source, the Interrupt Vectors are not used, and regular program code can
be placed at these locations. The circuit diagram in Figure 23 shows the reset logic.
Table 3 defines the timing and electrical parameters of the reset circuitry.
Figure 23. Reset Logic
Note: 1. The Power-on Reset will not work unless the supply voltage has been below V
POT
(falling).
The user can select the start-up time according to typical Oscillator start-up. The num-
ber 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 74.
Table 3. Reset Characteristics (V
CC
= 5.0V)
Symbol Parameter Min Typ Max Units
V
POT
(1)
Power-on Reset Threshold Voltage (rising) 1.0 1.4 1.8 V
Power-on Reset Threshold Voltage (falling) 0.4 0.6 0.8 V
V
RST
RESET Pin Threshold Voltage – 0.85 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
0.25 0.28 0.31 ms