Rainbow Electronics AT90S1200 - Manual (Page 13)

13
AT90S1200
0838H–AVR–03/02
Figure 13. Reset Logic
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 13, 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 14).
Figure 14. MCU Start-up, RESET Tied to V
CC
.
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
Table 3. Reset Characteristics (V
CC
= 5.0V)
Symbol Parameter Min Typ Max Units
V
POT
(1)
Power-on Reset Threshold Voltage (rising) 0.8 1.2 1.6 V
Power-on Reset Threshold Voltage (falling) 0.2 0.4 0.6 V
V
RST
Pin Threshold Voltage – – 0.85 V
CC
V
t
POR
Power-on Reset Period 2.0 3.0 4.0 ms
t
TOUT
Reset Delay Time-out Period (The Time-out
period equals 16K WDT cycles. See “Typical
Characteristics” on page 51. for typical WDT
frequency at different voltages).
11.0 16.0 21.0 ms
V
CC
Power-on Reset
Circuit
RESET
POR
100 - 500K
Reset Circuit
Watchdog
Timer
On-chip
RC Oscillator
Counter Reset
14-stage Ripple Counter
Time-out
S Q
R Q
Inter
nal Reset
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST