DS1814/DS1819 5V and 3.3V MicroMonitor www.maxim-ic.com FEATURES PIN ASSIGNMENT Halts and restarts an out-of-control 32 1 microprocessor Holds microprocessor in check during power transients Automatically restarts microprocessor after power failure 45 Monitors pushbutton for external override SOT-23-5 PACKAGE Accurate 5%, 10%, or 20% (3V only) See Mech. Drawing microprocessor power monitoring Eliminates the need for discrete components 20% tolerance for use with 3.0V systems PIN DESCRIPTION Pin- and function-compatible with the DS181xA MAX823/24/25 products 1 RST Active Low Reset Output Low cost SOT-23-5 package 2 GND Ground Industrial temperature range: -40 C to +85 C 3 PBRST Pushbutton Reset Input 4 ST Strobe Input DEVICE RST WD RST PBRST 5 VCC Power Supply DS181xA X X X DS181xB DS181xB X X X 1 RST Active Low Reset Output DS181xC X X X 2 GND Ground x =DS1814 for 5V devices 3 RST Active High Reset Output x =DS1819 for 3V devices 4 ST Strobe Input 5 VCC Power Supply DS181xC 1 RST Active Low Reset Output 2 GND Ground 3 RST Active High Reset Output 4 PBRST Pushbutton Reset Input 5 VCC 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 temperature- compensated reference and comparator circuit monitor the status of V . When an out-of-tolerance CC condition occurs, an internal power-fail signal is generated which forces the reset(s) to an active state. When V returns to an in-tolerance condition, the reset signal is kept in the active state for a minimum of CC 140ms to allow the power supply and processor to stabilize. 1 of 8 122001DS1814/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 V falls below a preset level, a comparator outputs the signal RST (or CC RST). RST (or RST) are excellent control signals for a microprocessor, as processing is stopped at the last possible moment of valid V . On power-up, RST (or RST) are kept active for a minimum of 140ms CC 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