MAX6319LHUK44B-T中文资料(8)

5-Pin µP Supervisory Circuits with Watchdog and Manual ResetMAX6316–MAX6322

Figure 4. MAX6316/MAX6318MH/MAX6319MH Bidirectional Reset Output Functional Diagram

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5-Pin µP Supervisory Circuits with

Watchdog and Manual Reset

P-channel transistor on-resistance quickly chargesregister a logic low (see Electrical Characteristics), andstray capacitance on the reset line, allowing RESETtosmall enough to register a logic high while supplying alltransition from low to high within the required two elec-input current and leakage paths connected to the RESETtronic-clock cycles, even with several devices on theline. A 10kΩpullup is sufficient in most applications.

reset line. This process occurs regardless of whetherthe reset was caused by VCCdipping below the resetManual-Reset Input

threshold, the watchdog timing out, MRbeing asserted,The MAX6316_/MAX6317H/MAX6319_H/MAX6320P/or the µP or other device asserting RESET. The parts doMAX6322HP feature a manual reset input. A logic low onnot require an external pullup. To minimize supply cur-MRasserts a reset. After MRtransitions low to high, resetrent consumption, the internal 4.7kΩpullup resistor dis-remains asserted for the duration of the reset timeout peri-connects from the supply whenever the MAX6316M/od (tRP). The MRinput is connected to VCCthrough anMAX6318MH/MAX6319MH assert reset.

internal 52kΩpullup resistor and therefore can be leftunconnected when not in use. MRcan be driven withOpen-Drain RESETOutput

TTL-logic levels in 5V systems, with CMOS-logic levels inThe MAX6320P/MAX6321HP/MAX6322HP have an3V systems, or with open-drain or open-collector outputactive-low, open-drain reset output. This output struc-devices. A normally-open momentary switch from MRtoture will sink current when RESETis asserted. Connectground can also be used; it requires no externala pullup resistor from RESETto any supply voltage updebouncing circuitry. MRis designed to reject fast, to 6V (Figure 6). Select a resistor value large enough to

negative-going transients (typically 100ns pulses). A0.1µF capacitor from MRto ground provides additionalnoise immunity.

The MRinput pin is equipped with internal ESD-protectioncircuitry that may become forward biased. Should MRbedriven by voltages higher than VCC, excessive currentwould be drawn, which would damage the part. Forexample, assume that MRis driven by a +5V supply otherthan VCC. If VCCdrops lower than +4.7V, MR’s absolute maximum rating is violated [-0.3V to (VCC+ 0.3V)], andundesirable current flows through the ESD structure fromMRto VCC. To avoid this, use the same supply for MRasthe supply monitored by VCC. This guarantees that thevoltage at MRwill never exceed VCC.

Figure 5. Bidirectional RESETTiming Diagram

Watchdog Input

The MAX6316_/MAX6317H/MAX6318_H/MAX6320P/MAX6321HP feature a watchdog circuit that monitorsthe µP’s activity. If the µP does not toggle the watchdoginput (WDI) within the watchdog timeout period (tWD),reset asserts. The internal watchdog timer is cleared byreset or by a transition at WDI (which can detect pulsesas short as 50ns). The watchdog timer remains clearedwhile reset is asserted. Once reset is released, thetimer begins counting again (Figure 7).

The WDI input is designed for a three-stated outputdevice with a 10µA maximum leakage current and thecapability of driving a maximum capacitive load of 200pF.The three-state device must be able to source and sink atleast 200µA when active. Disable the watchdog functionby leaving WDI unconnected or by three-stating the driverconnected to WDI. When the watchdog timer is left openFigure 6. MAX6320P/MAX6321HP/MAX6322HP Open-Draincircuited, the timer is cleared internally at intervals equalRESETOutput Allows Use with Multiple Supplies

to 7/8 of the watchdog period.

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MAX6316–MAX6322