PT7M7809 Datasheet, PDF (6/12 Pages) Pericom Semiconductor Corporation – Precision supply-voltage monitor

PT7M7803/7809-7812/7823-7825
µP Supervisor Circuits
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Block Diagram
WDI
MR
Vcc
PFI
Watchdog
Transition Detector
V _cc
Watchdog Timer
Timebase for Reset
& Watchdog
Reset Generator
V_RST
1.25V
RESET
(RESET)
PFO
Functional Description
Reset Output
A microprocessor (µP) reset input starts the µP in a known state. Whenever the µP is in an unknown state, it should be held in
reset. The supervisory circuits assert reset during power-up and prevent code execution errors during power-down or brownout
conditions.
On power-up, once Vcc reaches about 1.0V, RESET is a guaranteed logic low of 0.4V or less. As Vcc rises, RESET stays low.
When Vcc rises above the reset threshold, an internal timer releases RESET after about 200ms. RESET pulses low whenever Vcc
drops below the reset threshold, i.e. brownout condition. If brownout occurs in the middle of a previously initiated reset pulse, the
pulse continues for at least another 200ms. On power-down, once Vcc falls below the reset threshold, RESET stays low and is
guaranteed to be 0.4V or less until Vcc drops below 1.0V. Watchdog Timing Diagram shows the timing relationship.
The active-high RESET output is simply the inverse of the RESET output, and is guaranteed to be valid with Vcc down to 1.0V.
Watchdog Timer
The PT7M78xx watchdog circuit monitors the µP activity. If the µP does not toggle the watch-dog input (WDI) within 1.6s, reset
asserts. As long as reset is asserted or the WDI input is toggled, the watchdog timer will stay clear and will not count. As soon as
reset is released, the timer will start counting. WDI input pulses as short as 50ns can be detected.
Disable the watchdog function by leaving WDI unconnected or by three-stating driver connected to WDI.
Manual Reset
The manual-reset input (MR) allows reset to be triggered by a push button switch. MR has an internal pullup resistor, so it can be
left open when not used.
Power-Fail Comparator
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.
11-0012
PT0109S-15
10/14/10
6