FEATURES
RESET Output
The SP705-708/813L series provides four
key functions:
1. A reset output during power-up, power-down
and brownout conditions.
2. An independent watchdog output that goes
LOW if the watchdog input has not been toggled
within 1.6 seconds.
3. A 1.25V threshold detector for power-fail
warning, low battery detection, or monitoring a
power supply other than +5V.
A microprocessor's reset input starts the µP
in a known state. The SP705-708/813L
series asserts reset during power-up and
prevents code execution errors during power-
down or brownout conditions.
On power-up, once VCC reaches 1.1V, RESET
is aguaranteedlogicLOWof0.4Vorless. AsVCC
rises, RESET stays LOW. When VCC rises
above the reset threshold, an internal timer re-
leases RESET after 200ms. RESET pulses
LOWwheneverVCC dipsbelowtheresetthresh-
old, such as in a brownout condition. When a
brownout condition occurs in the middle of a
previously initiated reset pulse, the pulse con-
tinues for at least another 140ms. On power-
down, once VCC falls below the reset threshold,
RESETstaysLOWandisguaranteedtobe0.4V
or less until VCC drops below 1.1V.
4. An active-LOW manual-reset that allows
RESET to be triggered by a pushbutton switch.
The SP707/708 devices are the same as the
SP705/706 devices except for the active-HIGH
RESET substitution of the watchdog timer. The
SP813L is the same as the SP705 except an
active-HIGH RESET is provided rather than an
active-LOW RESET. The SP705/707/813L
devices generate a reset when the supply voltage
drops below 4.65V. The SP706/708
The SP707/708/813L active-HIGH
devices generate a reset below 4.40V.
RESET output is simply the complement of the
RESEToutputandisguaranteedtobevalidwith
VCC down to 1.1V. Some µPs, such as Intel's
80C51, require an active-HIGH reset pulse.
The SP705-708/813L series is ideally
suited for applications in automotive systems,
intelligent instruments, and battery-powered
computersandcontrollers.TheSP705-708/813L
series is ideally applied in environments
wheremonitoringofpowersupplytoaµPandits
related components is critical.
Watchdog Timer
The SP705/706/813L watchdog circuit
monitors the µP's activity. If the µP does not
toggle the watchdog input (WDI) within 1.6
seconds and WDI is not tri-stated, WDO goes
LOW. AslongasRESETisassertedortheWDI
input is tri-stated, the watchdog timer will stay
cleared and will not count. As soon as RESET
is released and WDI is driven HIGH or LOW,
the timer will start counting. Pulses as short as
50ns can be detected.
THEORY OF OPERATION
The SP705-708/813L series is a
microprocessor (µP) supervisory circuit that
monitors the power supplied to digital circuits
such as microprocessors, microcontrollers, or
memory. The series is an ideal solution for
portable, battery-powered equipment that
requirespowersupplymonitoring.Implementing
thisserieswillreducethenumberofcomponents
andoverallcomplexity. Thewatchdogfunctions
of this product family will continuously oversee
theoperationalstatusofasystem. Theoperational
features and benefits of the SP705-708/813L
series are described in more detail below.
June 2008 Rev C
SP705 Low Power Microprocessor Supervisory Circuits
© 2008 Exar Corporation
9
EXAR [ EXAR CORPORATION ]
