November 2006
rev 1.6
ASM1232LP/LPS
When PBRST is held LOW for the minimum time tPB, both
power-up after the supply voltage returns to an in-tolerance
condition, the reset signal remains active for 250ms
minimum, allowing the power supply and system
microprocessor to stabilize. ST pulses as short as 20ns can
be detected.
resets become active and remain active for a minimum time
period of 250ms after PBRST returns HIGH.
The debounced input is guaranteed to recognize pulses
greater than 20ms. No external pull-up resistor is required,
since PBRST is pulled HIGH by an internal 40kΩ resistor.
Valid
Strobe
Valid
Strobe
Invalid
Strobe
ST
t
ST
The PBRST can be driven from a TTL or CMOS logic line or
shorted to ground with a mechanical switch.
t
RST
t
(min)
t
(max)
TD
TD
t
RESET
PB
PBRST
Note: ST is ignored whenever a reset is active
V
IH
t
Figure 5: Timing Diagram: Strobe Input
PDLY
V
IL
Timeouts periods of approximately 150ms, 610ms or
1,200ms are selected through the TD pin.
t
RST
RESET
RESET
VOH
VOL
Watchdog Time-out Period
TD Voltage level
(ms)
Min
Nom
Max
Figure 3: Timing Diagram: Pushbutton Reset
GND
62.5
250
500
150
610
250
1000
2000
Floating
VCC
5V
1200
ASM1232LP/LPS
1
2
8
7
V
CC
PBRST
TD
The watchdog timer can not be disabled. It must be strobed
with a high-to-low transition to avoid watchdog timeout and
reset.
I/O
ST
µP
3
4
6
TOL
RESET
RESET
5
GND
RESET
5V
ASM1232 LP/LPS
MREQ
Figure 4: Application Circuit: Pushbutton Reset
1
2
8
7
V
CC
PBRST
10kΩ
T
D
ST
µP
Watchdog Timer and ST Input
Decoder
3
4
6
A watchdog timer stops and restarts a microprocessor that is
“hung-up”. The µP must toggle the ST input within a set
period (as selectable through TD input) to verify proper
software execution. If the ST is not toggled low within the
minimum timeout period, reset signals become active. In
TOL
RESET
Address
Bus
RESET
5
GND
Figure 6: Application Circuit: Watchdog Timer
4 of 10
5V µP Power Supply Monitor and Reset Circuit
Notice: The information in this document is subject to change without notice
PULSECORE [ PulseCore Semiconductor ]
