《微机技术与仪器系统设计》课程教学资源（芯片手册）MAX813L-看门狗芯片.pdf_大学文库

MAX705–MAX708/MAX813L Low-Cost, µP Supervisory Circuits _ Maxim Integrated Products 1 TOP VIEW 1234 8765 WDO RESET (RESET) WDI PFI PFO GND VCC MR DIP/SO MAX705 MAX706 MAX813L 1234 8765 RESET RESET N.C. PFI PFO GND VCC MR DIP/SO MAX707 MAX708 ( ) ARE FOR MAX813L ONLY. Pin Configurations continued at end of data sheet. _Pin Configurations VCC PFI MR PFO WDO RESET UNREGULATED DC PUSHBUTTON SWITCH PFI I/O LINE INTERRUPT µP MAX705 MAX706 MAX813L MAX667 +5V DC LINEAR REGULATOR VCC RESET NMI _Typical Operating Circuit Call toll free 1-800-998-8800 for free samples or literature. 19-4334; Rev. 5; 9/95 _General Description The MAX705-MAX708/MAX813L microprocessor (µP) supervisory circuits reduce the complexity and number of components required to monitor power-supply and battery functions in µP systems. These devices significantly improve system reliability and accuracy compared to separate ICs or discrete components. The MAX705/MAX706/MAX813L provide four 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 for monitoring a power supply other than +5V. 4) An active-low manual-reset input. The MAX707/MAX708 are the same as the MAX705/ MAX706, except an active-high reset is substituted for the watchdog timer. The MAX813L is the same as the MAX705, except RESET is provided instead of RESET . Two supply-voltage monitor levels are available: The MAX705/MAX707/MAX813L generate a reset pulse when the supply voltage drops below 4.65V, while the MAX706/MAX708 generate a reset pulse below 4.40V. All four parts are available in 8-pin DIP, SO and µMAX packages. _Applications Computers Controllers Intelligent Instruments Automotive Systems Critical µP Power Monitoring _Features ♦ µMAX Package: Smallest 8-Pin SO ♦ Guaranteed RESET Valid at VCC = 1V ♦ Precision Supply-Voltage Monitor 4.65V in MAX705/MAX707/MAX813L 4.40V in MAX706/MAX708 ♦ 200ms Reset Pulse Width ♦ Debounced TTL/CMOS-Compatible Manual-Reset Input ♦ Independent Watchdog Timer—1.6sec Timeout (MAX705/MAX706) ♦ Active-High Reset Output (MAX707/MAX708/MAX813L) ♦ Voltage Monitor for Power-Fail or Low-Battery Warning _Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX705CPA 8 Plastic DIP MAX705CSA 0°C to +70°C 8 SO MAX705C/D 0°C to +70°C Dice* Ordering Information continued at end of data sheet. * Dice are specified at TA = +25°C. **Contact factory for availability and processing to MIL-STD-883. 0°C to +70°C MAX705CUA 0°C to +70°C 8 µMAX

MAX705–MAX708/MAX813L Low-Cost, µP Supervisory Circuits 2 _ Terminal Voltage (with respect to GND) VCC . -0.3V to 6.0V All Other Inputs (Note 1) . -0.3V to (VCC + 0.3V) Input Current VCC . 20mA GND . 20mA Output Current (all outputs) . 20mA Continuous Power Dissipation Plastic DIP (derate 9.09mW/°C above +70°C) . 727mW SO (derate 5.88mW/°C above +70°C) . 471mW µMAX (derate 4.10mW/°C above +70°C) . 330mW CERDIP (derate 8.00mW/°C above +70°C) . 640mW Operating Temperature Ranges MAX70_C_, MAX813LC_ . 0°C to +70°C MAX70_E_, MAX813LE_ . -40°C to +85°C MAX70_MJA . -55°C to +125°C Storage Temperature Range . -65°C to +160°C Lead Temperature (soldering, 10sec) . +300°C ELECTRICAL CHARACTERISTICS (VCC = 4.75V to 5.5V for MAX705/MAX707/MAX813L, VCC = 4.5V to 5.5V for MAX706/MAX708, TA = TMIN to TMAX, unless otherwise noted.) Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Operating Voltage Range VCC MAX70_C 1.0 5.5 MAX813LC 1.1 5.5 V MAX70_E/M, MAX813LE/M 1.2 5.5 Reset Threshold (Note 2) VRT MAX705, MAX707, MAX813L 4.50 4.65 4.75 MAX706, MAX708 4.25 4.40 4.50 40 mV Reset Pulse Width (Note 2) tRS 140 200 280 ms RESET Output Voltage ISOURCE = 800µA VCC - 1.5 V ABSOLUTE MAXIMUM RATINGS Note 1: The input voltage limits on PFI and MR can be exceeded if the input current is less than 10mA. Reset Threshold Hysteresis (Note 2) ISINK = 3.2mA 0.4 0.3 0.3 RESET Output Voltage MAX707, MAX708, ISOURCE = 800µA V MAX707, MAX708, ISINK = 1.2mA 0.4 0.8 0.9 MAX813L VCC - 1.5 0.4 Watchdog Timeout Period tWD MAX705, MAX706, MAX813L 1.00 1.60 2.25 sec WDI Pulse Width tWP VIL = 0.4V, VIH = (VCC) (0.8) 50 ns WDI Input Threshold 0.8 V 3.5 MAX70_C, VCC = 1V, ISINK = 50µA MAX70_E/M, VCC = 1.2V, ISINK = 100µA MAX813LC, ISOURCE = 4µA, VCC = 1.1V MAX813LE/M, ISOURCE = 4µA, VCC = 1.2V ISOURCE = 800µA ISINK = 3.2mA Low High WDI Input Current MAX705, MAX706, MAX813L, WDI = VCC 50 150 µA MAX705, MAX706, MAX813L, WDI = 0V -150 -50 WDO Output Voltage VCC - 1.5 V 0.4 MAX705, MAX706, MAX813L, VCC = 5V MAX705, MAX706, MAX813L, ISOURCE = 800µA MAX705, MAX706, MAX813L, ISINK = 1.2mA V VCC - 1.5 MAX707C, MAX708C 50 350 MAX707E/M, MAX708E/M 50 500 Supply Current ISUPPLY MAX705C, MAX706C, MAX813LC 150 350 µA MAX705E/M, MAX706E/M, MAX813LE/M 150 500

MAX705–MAX708/MAX813L Low-Cost, µP Supervisory Circuits _ 5 _Pin Description Watchdog Output pulls low when the internal watchdog timer finishes its 1.6sec count and does not go high again until the watchdog is cleared. WDO also goes low during low-line conditions. Whenever VCC is below the reset threshold, WDO stays low; however, unlike RESET, WDO does not have a minimum pulse width. As soon as VCC rises above the reset threshold, WDO goes high with no delay. Active-High Reset Output is the inverse of RESET . Whenever RESET is high, RESET is low, and vice versa (Figure 2). The MAX813L has a RESET output only. Active-Low Reset Output pulses low for 200ms when triggered, and stays low whenever VCC is below the reset threshold (4.65V in the MAX705 and 4.40V in the MAX706). It remains low for 200ms after VCC rises above the reset threshold or MR goes from low to high (Figure 3). A watchdog timeout will not trigger RESET unless WDO is connected to MR . Watchdog Input. If WDI remains high or low for 1.6sec, the internal watchdog timer runs out and WDO goes low (Figure 1). Floating WDI or connecting WDI to a high-impedance three-state buffer disables the watchdog feature. The internal watchdog timer clears whenever reset is asserted, WDI is threestated, or WDI sees a rising or falling edge. Power-Fail Output goes low and sinks current when PFI is less than 1.25V; otherwise PFO stays high. Power-Fail Voltage Monitor Input. When PFI is less than 1.25V, PFO goes low. Connect PFI to GND or VCC when not used. MAX705/MAX706 FUNCTION - 8 7 RESET 8 - 8 WDO 7 7 - RESET - 6 - N.C. No Connect 6 - 6 WDI 5 5 5 PFO 4 4 4 PFI 3 3 3 GND 0V Ground Reference for all signals 2 2 2 VCC +5V Supply Input NAME DIP/SO DIP/SO DIP/SO MAX707/MAX708 PIN -21-87654 µMAX 2-1-7654 µMAX 12-87654 µMAX MAX813L Manual-Reset Input triggers a reset pulse when pulled below 0.8V. This active-low input has an internal 250µA pull-up current. It can be driven from a TTL or CMOS logic line as well as shorted to ground with a switch. 1 3 1 3 1 3 MR

MAX705–MAX708/MAX813L _Detailed Description Reset Output A microprocessor’s (µP’s) reset input starts the µP in a known state. Whenever the µP is in an unknown state, it should be held in reset. The MAX705-MAX708/MAX813L assert reset during power-up and prevent code execution errors during power-down or brownout conditions. On power-up, once VCC reaches 1V, 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 dips 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 140ms. 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 1V. The MAX707/MAX708/MAX813L active-high RESET output is simply the complement of the RESET output, and is guaranteed to be valid with VCC down to 1.1V. Some µPs, such as Intel’s 80C51, require an active-high reset pulse. Watchdog Timer The MAX705/MAX706/MAX813L watchdog circuit monitors the µP’s activity. If the µP does not toggle the watchdog input (WDI) within 1.6sec and WDI is not three-stated, WDO goes low. As long as RESET is asserted or the WDI input is three-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 VCC drops below the reset threshold, WDO will go low whether or not the watchdog timer has timed out yet. Normally this would trigger an NMI interrupt, but RESET goes low simultaneously, and thus overrides the NMI interrupt. If WDI is left unconnected, WDO can be used as a lowline output. Since floating WDI disables the internal timer, WDO goes low only when VCC falls below the reset threshold, thus functioning as a low-line output. The MAX705/MAX706 have a watchdog timer and a RESET output. The MAX707/MAX708 have both activehigh and active-low reset outputs. The MAX813L has both an active-high reset output and a watchdog timer. Manual Reset The manual-reset input (MR) allows reset to be triggered by a pushbutton switch. The switch is effectively debounced by the 140ms minimum reset pulse width. MR is TTL/CMOS logic compatible, so it can be driven by an external logic line. MR can be used to force a watchdog timeout to generate a reset pulse in the MAX705/MAX706/MAX813L. Simply connect WDO to MR . Low-Cost, µP Supervisory Circuits 6 _ WATCHDOG TRANSITION DETECTOR WATCHDOG TIMER TIMEBASE FOR RESET AND WATCHDOG RESET GENERATOR VCC 250µ A 8 WDO 7 RESET (RESET) 5 PFO 3 GND 1.25V 4.65V* * 4.40V FOR MAX7O6. ( ) ARE FOR MAX813L ONLY. 6 WDI 1 MR 2 VCC 4 PFI MAX705 MAX706 MAX813L Figure 1. MAX705/MAX706/MAX813L Block Diagram RESET GENERATOR VCC 250µ A 8 RESET 7 RESET 5 PFO 3 GND 1.25V 4.65V* * 4.40V FOR MAX7O6. 1 MR 2 VCC 4 PFI MAX707 MAX708 Figure 2. MAX707/MAX708 Block Diagram

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. To build an early-warning circuit for power failure, connect the PFI pin to a voltage divider (see Typical Operating Circuit). Choose the voltage divider ratio so that the voltage at PFI falls below 1.25V just before the +5V regulator drops out. Use PFO to interrupt the µP so it can prepare for an orderly power-down. _Applications Information Ensuring a Valid RESET Output Down to VCC = 0V When VCC falls below 1V, the MAX705-MAX708 RESET output no longer sinks current—it becomes an open circuit. High-impedance CMOS logic inputs can drift to undetermined voltages if left undriven. If a pull-down resistor is added to the RESET pin as shown in Figure 5, any stray charge or leakage currents will be drained to ground, holding RESET low. Resistor value (R1) is not critical. It should be about 100kΩ, large enough not to load RESET and small enough to pull RESET to ground. Monitoring Voltages Other Than the Unregulated DC Input Monitor voltages other than the unregulated DC by connecting a voltage divider to PFI and adjusting the ratio appropriately. If required, add hysteresis by connecting a resistor (with a value approximately 10 times the sum of the two resistors in the potential divider network) between PFI and PFO. A capacitor between PFI and GND will reduce the power-fail circuit’s sensitivity to high-frequency noise on the line being monitored. RESET can be asserted on other voltages in addition to the +5V VCC line. Connect PFO to MR to initiate a RESET pulse when PFI drops below 1.25V. Figure 6 shows the MAX705-MAX708 configured to assert RESET when the +5V supply falls below the reset threshold, or when the +12V supply falls below approximately 11V. Monitoring a Negative Voltage The power-fail comparator can also monitor a negative supply rail (Figure 7). When the negative rail is good (a negative voltage of large magnitude), PFO is low, and when the negative rail is degraded (a negative voltage of lesser magnitude), PFO is high. By adding the resistors and transistor as shown, a high PFO triggers reset. As long as PFO remains high, the MAX705- MAX708/MAX813L will keep reset asserted (RESET = low, RESET = high). Note that this circuit’s accuracy depends on the PFI threshold tolerance, the VCC line, and the resistors. MAX705–MAX708/MAX813L Low-Cost, µP Supervisory Circuits _ 7 tWP tWD tWD tWD +5V 0V WDI +5V 0V WDO +5V 0V RESET +5V 0V (RESET) tRS ( ) ARE FOR MAX813L ONLY. RESET EXTERNALLY TRIGGERED BY MR Figure 3. MAX705/MAX706/MAX813L Watchdog TIming Figure 4. MAX705/MAX706 RESET, MR, and WDO Timing with WDI Three-Stated. The MAX707/MAX708/MAX813L RESET output is the inverse of RESET shown. tRS tRS +5V 0V RESET WDO +5V 0V MR tMR MR EXTERNALLY DRIVEN LOW +5V 0V VCC VRT VRT tMD