DS1819CR-20-U+

DS1814/DS1819 5V and 3.3V MicroMonitor www.maxim-ic.com FEATURES § § § § § § § § § § PIN ASSIGNMENT Halts and restarts an out-of-control microprocessor Holds microprocessor in check during power transients Automatically restarts microprocessor after power failure Monitors pushbutton for external override Accurate 5%, 10%, or 20% (3V only) microprocessor power monitoring Eliminates the need for discrete components 20% tolerance for use with 3.0V systems Pin- and function-compatible with the MAX823/24/25 products Low cost SOT-23-5 package Industrial temperature range: -40°C to +85°C DEVICE RST RST DS181xA X DS181xB X X DS181xC X X x = "DS1814" for 5V devices x = "DS1819" for 3V devices PBRST WD X X X 3 4 2 1 5 SOT-23-5 PACKAGE See Mech. Drawing PIN DESCRIPTION X DS181xA 1 RST 2 GND PBRST 3 4 ST 5 VCC Active Low Reset Output Ground Pushbutton Reset Input Strobe Input Power Supply DS181xB 1 RST 2 GND 3 RST 4 ST 5 VCC Active Low Reset Output Ground Active High Reset Output Strobe Input Power Supply DS181xC 1 RST 2 GND 3 RST 4 PBRST 5 VCC Active Low Reset Output Ground Active High Reset Output Pushbutton Reset Input Power Supply DESCRIPTION The DS1814 5-Volt MicroMonitor and the DS1819 3.3V MicroMonitor monitor three vital conditions for a microprocessor: power supply, software execution, and external override. First, a precision temperaturecompensated reference and comparator circuit monitor the status of VCC. When an out-of-tolerance condition occurs, an internal power-fail signal is generated which forces the reset(s) to an active state. When VCC returns to an in-tolerance condition, the reset signal is kept in the active state for a minimum of 140ms to allow the power supply and processor to stabilize. 1 of 8 122001 DS1814/DS1819 The DS1814/DS1819 ‘A’ and ‘C’ perform pushbutton reset control. They debounce the pushbutton input and generate an active reset pulse width of 140ms minimum. The DS1814/9 ‘A’ and ‘B’ versions perform a watchdog function. The watchdog is an internal timer that forces the reset signals to the active state if the strobe input does not change state every 1.12 seconds. The watchdog timer function can be disabled by leaving the watchdog strobe input disconnected. OPERATION Power Monitor The DS1814 detects out-of-tolerance power supply conditions and warns a processor-based system of impending power failure. When VCC falls below a preset level, a comparator outputs the signal RST (or RST). RST (or RST) are excellent control signals for a microprocessor, as processing is stopped at the last possible moment of valid VCC. On power-up, RST (or RST) are kept active for a minimum of 140ms to allow the power supply and processor to stabilize. Pushbutton Reset The DS1814 ‘A’ and ‘C’ provide an input pin for direct connection to a pushbutton reset (see Figure 2). The pushbutton reset input requires an active low signal. Internally, this input is debounced and timed such that a RST (or RST) signal of at least 140ms minimum will be generated. The 140ms delay commences as the pushbutton reset input is released from the low level (see Figure 3). Watchdog Timer The DS1814/DS1819 ‘A’ and ‘B’ versions watchdog timer function forces RST (and RST) signal(s) active when the ST input is not clocked within the 1.12 second watchdog time-out period. Time-out of the watchdog starts when RST (and RST) become(s) inactive. If a transition occurs on the ST input pin prior to time-out, the watchdog timer is reset and begins to time out again. If the watchdog timer is allowed to time out, then the reset output(s) will go active for 140ms. The watchdog can be disabled by floating (or tri-stating) the ST input. The ST input can be derived from many microprocessor outputs. The most typical signals used are the microprocessor data I/O signals and control signals. When the microprocessor functions normally, these signals would as a matter of routine cause the watchdog to be reset prior to time-out. To guarantee that the watchdog timer does not time out, a transition must occur at or less than 1.12 seconds. A typical circuit example is shown in Figure 4. The DS1814/19 watchdog function can be disabled at any time by tri-starting the strobe input. A sample circuit to disable the watchdog is shown in Figure 6. 2 of 8 DS1814/DS1819 DS1814/DS1819 BLOCK DIAGRAM Figure 1 *Notes: ST available on'A' and 'B' versions PBRST available on 'A' and 'C' versions RST available on 'B' and 'C' versions PUSHBUTTON RESET Figure 2 TIMING DIAGRAM: PUSHBUTTON RESET Figure 3 3 of 8 DS1814/DS1819 WATCHDOG TIMER Figure 4 TIMING DIAGRAM: STROBE INPUT Figure 5 WATCHDOG DISABLE CIRCUIT Figure 6 TIMING DIAGRAM: POWER-DOWN Figure 7 4 of 8 DS1814/DS1819 TIMING DIAGRAM: POWER-UP Figure 8 RST VALID TO 0 VOLTS VCC Figure 9 OUTPUT VALID CONDITIONS The RST output uses a push-pull output which can maintain a valid output down to 1.0V VCC. To sink current below 1.0V a resistor can be connected form RST to Ground (see Figure 9.) This arrangement will maintain a valid value on RST during both power-up and -down but will draw current when RST is in the high state. A value of about 100kW should be adequate in most situations. The output with a resistor pull-down can maintain a valid reset down to VCC equal to 0V. 5 of 8 DS1814/DS1819 ABSOLUTE MAXIMUM RATINGS* Voltage on VCC Pin Relative to Ground Voltage on I/O Relative to Ground Operating Temperature Range Storage Temperature Range Soldering Temperature * -0.5V to +7.0V -0.5V to VCC + 0.5V -40°C to +85°C -55°C to +125°C 260°C for 10 seconds This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. RECOMMENDED DC OPERATING CONDITIONS PARAMETER Supply Voltage SYMBOL VCC MIN 1.0 ST and PBRST Input High Level VIH 0.8 VCC ST and PBRST Input Low Level VIL TYP MAX 5.5 0.3 VCC DC ELECTRICAL CHARACTERISTICS PARAMETER (-40°C to +85°C) UNITS V NOTES 1 V 1 V 1 (-40°C to +85°C; VCC = 1.2V to 5.5V) SYMBOL MIN VCC Trip Point DS1814-5 VCCTP VCC Trip Point DS1814-10 TYP MAX UNITS NOTES 4.50 4.75 V 1 VCCTP 4.25 4.50 V 1 VCC Trip Point DS1819-5 VCCTP 3.00 3.15 V 1 VCC Trip Point DS1819-10 VCCTP 2.85 3.00 V 1 VCC Trip Point DS1819-20 VCCTP 2.55 2.70 V 1 Reset Trip Point Hysteresis (DS1814) VRH 10 mV Reset Trip Point Hysteresis (DS1819) VRH 5 mV Output Current @ 2.4V IOH 350 mA 3 Output Current @ 0.4V IOL 3.2 6 mA 3 Output Voltage @ -30 mA VOH VCC*.8 VCC*.9 V 3 Operating Current DS1814 ICC 10 15 mA 4 Operating Current DS1819 ICC 8 12 mA 5 120 160 mA 6 mA 6 ST Input Current ST = VCC, Time Averaged -20 ST Input Current ST = GND, Time Averaged 6 of 8 -15 DS1814/DS1819 AC ELECTRICAL CHARACTERISTICS PARAMETER (-40°C to +85°C; VCC = 1.2V to 5.5V) SYMBOL tPB MIN 1 TYP Reset Active Time tRST 140 200 ST Pulse Width tST 50 Watchdog Time-out tTD 1.12 VCC Detect to RST and RST tRPD PBRST = VIL VCC Slew Rate (rising edge) MAX ms 280 tRPU PBRST Stable Low to RST and RST tPDLY 140 1.60 2.40 s 30 40 ms Output Capacitance 8 9 V/ms 200 280 500 ms ns CAPACITANCE PARAMETER Input Capacitance NOTES 7 ms ns 12.5 VCC Detect to RST and RST UNITS (tA = 25°C) SYMBOL CIN MIN TYP MAX 5 UNITS pF 7 pF COUT NOTES NOTES: 1) All voltages are referenced to ground. 2) PBRST is internally pulled up to VCC with an internal impedance of 50kW typical. 3) Measured with VCC ³ 2.7V. 4) Measured with outputs open, VCC £ 5.5V, and all inputs at VCC or Ground, except the ST input at