SP706P Datasheet, PDF (10/18 Pages) Sipex Corporation – +3.0V/+3.3V Low Power Microprocessor Supervisory Circuits

FEATURES
The SP706P/R/S/T-SP708R/S/T 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 sec.
3. A 1.25V threshold detector for power-fail
warning, low battery detection, or monitoring a
power supply other than +3.3V/+3.0V.
4. An active-LOW manual-reset that allows
RESET to be triggered by a pushbutton switch.
The SP706R/S/T devices are the same as the
SP708R/S/T devices except for the active-HIGH
RESET substitution of the watchdog timer. The
SP706P device is the same as the SP706R de-
vice except an active-HIGH RESET is provided
rather than an active-LOW RESET.
THEORY OF OPERATION
The SP706P/R/S/T-SP708R/S/T series is a mi-
croprocessor (µP) supervisory circuit that moni-
tors the power supplied to digital circuits such
as microprocessors, microcontrollers, or
memory. The series is an ideal solution for
portable, battery-powered equipment that re-
quires power supply monitoring. Implementing
this series will reduce the number of compo-
nents and overall complexity of a system. The
watchdog functions of this product family will
continuously oversee the operational status of a
system. The operational features and benefits of
the SP706P/R/S/T-SP708R/S/T series are de-
scribed, in more detail, below.
the reset threshold, an internal timer releases
RESET after 200ms. RESET pulses LOW when-
ever V_CC dips below the reset threshold, such as
in a brownout condition. When a brownout
condition occurs in the middle of a previously
initiated reset pulse, the pulse continues for at
least another 140ms. During power-down, once
V_CC falls below the reset threshold, RESET
stays LOW and is guaranteed to be 0.4V or less
until V_CC drops below 1V.
The active-HIGH RESET output is simply
the complement of the RESET output and is
guaranteed to be valid with V down to 1.1V.
CC
Some µPs, such as Intel's 80C51, require an
active-HIGH reset pulse.
Watchdog Timer
The SP706P/R/S/T-SP708R/S/T series 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. As long as RESET is asserted or the WDI
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.
Typically, WDO will be connected to the
non-maskable interrupt input (NMI) of a µP.
When V_CC drops below the reset threshold, WDO
will go LOW independent of the current status
of the watchdog timer. Normally this would
trigger an NMI but RESET goes LOW simulta-
neously, and thus overrides the NMI.
RESET Output
A microprocessor's reset input starts the µP
in a known state. The SP706P/R/S/T-SP708R/
S/T series asserts reset during power-up and
prevents code execution errors during power-
down or brownout conditions.
If WDI is left unconnected, WDO can be used as
a low-line output. Since floating WDI disables
the internal timer, WDO goes LOW only when
V_CC falls below the reset threshold, thus
functioning as a low-line output.
Power-Fail Comparator
During power-up, once V reaches 1V, RESET
CC
is a guaranteed logic LOW of 0.4V or less. As
V_CC
rises, RESET stays LOW. When V rises above
CC
The power-fail comparator can be used for
various purposes because its output and
noninverting input are not internally connected.
The inverting input is internally connected to
a 1.25V reference.
Rev. 10-17-00
SP706 +3.0/ +3.3 Low Power Microprocessor Circuits
10
© Copyright 2000 Sipex Corporation