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2013-04-0003 PT0109-18 04/18/13
7
PT7M7803/7809-7812/7823-7825
μP Supervisor Circuits
Functional Description
Reset Output
A microprocessor (P) reset input starts the P in a known state. Whenever the P 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 P activity. If the P 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.
Typical Application Circuit
DC Linear
Regulator
Vcc
Supervisory
PFI Circuit WDI
P
Vcc
RESET
I/O Line
Interrupt
IN
RESET
PFOMR
P