ASM705, ASM706, ASM707, ASM708, ASM813L
http://onsemi.com
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Detailed Description
A proper reset input enables a microprocessor /
microcontroller to start in a known state. ASM70X and
ASM813L assert reset to prevent code execution errors
during power−up, power−down and brown−out conditions.
RESET/RESET Timing
The RESET/RESET signals are designed to start a mP/mC
in a known state or return the system to a known state.
The ASM707/708 have two reset outputs, one
active−HIGH RESET and one active−LOW RESET output.
The ASM813L has only an active−HIGH output. RESET is
simply the complement of RESET.
RESET is guaranteed to be LOW with VCC above 1.2 V.
During a power−up sequence, RESET remains low until
the supply rises above the threshold level, either 4.65 V or
4.40 V. RESET goes high approximately 200 ms after
crossing the threshold.
During power−down, RESET goes LOW as VCC falls
below the threshold level and is guaranteed to be under 0.4 V
with VCC above 1.2 V.
In a brownout situation where VCC falls below the
threshold level, RESET pulses low. If a brown−out occurs
during an already initiated reset, the pulse will continue for
a minimum of 140 ms.
Power Failure Detection with Auxiliary Comparator
All devices have an auxiliary comparator with 1.25 V trip
point and uncommitted output (PFO) and noninverting input
(PFI). This comparator can be used as a supply voltage
monitor with an external resistor voltage divider. The
attenuated voltage at PFI should be set just below the 1.25
threshold. As the supply level falls, PFI is reduced causing
the PFO output to transit LOW. Normally PFO interrupts the
processor so the system can be shut down in a controlled
manner.
Figure 3. WDI Three−state Operation
Manual Reset (MR)
The active−LOW manual reset input is pulled high by a
250 mA pull−up current and can be driven low by
CMOS/TTL logic or a mechanical switch to ground. An
external debounce circuit is unnecessary since the 140 ms
minimum reset time will debounce mechanical pushbutton
switches.
By connecting the watchdog output (WDO) and MR, a
watchdog timeout forces RESET to be generated. The
ASM813L should be used when an active−HIGH RESET is
required.
Watchdog Timer
The watchdog timer available on the ASM705/706/813L
monitors mP/mC activity. An output line on the processor is
used to toggle the WDI line. If this line is not toggled within
1.6 seconds, the internal timer puts the watchdog output,
WDO, into a LOW state. WDO will remain LOW until a
toggle is detected at WDI.
If WDI is floated or connected to a three−stated circuit, the
watchdog function is disabled, meaning, it is cleared and not
counting. The watchdog timer is also disabled if RESET is
asserted. When RESET becomes inactive and the WDI input
sees a high or low transition as short as 50 ns, the watchdog
timer will begin a 1.6 second countdown. Additional
transitions at WDI will reset the watchdog timer and initiate
a new countdown sequence.
WDO will also become LOW and remain so, whenever
the supply voltage, VCC, falls below the device threshold
level. WDO goes HIGH as soon as VCC transitions above the
threshold. There is no minimum pulse width for WDO as
there is for the RESET outputs. If WDI is floated, WDO
essentially acts as a low−power output indicator.
Figure 4. Watchdog Timing