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MAX16057ATT__+T Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX16057ATT__+T Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 16 page 125nA Supervisory Circuits with Capacitor- Adjustable Reset and Watchdog Timeouts _______________________________________________________________________________________ 7 Detailed Description The MAX16056–MAX16059 are ultra-low-current 125nA (typ) µP supervisory circuits that monitor a single sys- tem supply voltage. These devices assert an active-low reset signal whenever the VCC supply voltage drops below the factory-trimmed reset threshold, manual reset is pulled low, or the watchdog timer runs out (MAX16056/MAX16058). The reset output remains asserted for an adjustable reset timeout period after VCC rises above the reset threshold. The reset and watchdog delay periods are adjustable using external capacitors. RESET Output The MAX16056–MAX16059 µP supervisory circuits assert a reset to prevent code-execution errors during power- up, power-down, and brownout conditions. The reset output is guaranteed to be valid for VCC down to 1.1V. When VCC falls below the reset threshold, the RESET output asserts low. Once VCC exceeds the reset thresh- old plus the hysteresis, an internal timer keeps the reset output asserted for the capacitor-adjusted reset timeout period (tRP), then after this interval the reset output deasserts (see Figure 1). The reset function features immunity to power-supply voltage transients. Manual-Reset Input ( MR) Many µP-based products require manual-reset capabil- ity, allowing the operator, a test technician, or external logic circuitry to initiate a reset. The MAX16056– MAX16059 feature an MR input. A logic-low on MR asserts a reset. RESET remains asserted while MR is low and for the timeout period, tRP, after MR returns high. Connect MR to VCC if unused. MR can be driven with CMOS logic levels or with open-drain/collector out- puts (with a pullup resistor). Connect a normally open momentary switch from MR to GND and a resistor from MR to VCC to implement a manual-reset function; exter- nal debounce circuitry is not required. If MR is driven by long cables or the device is used in a noisy environ- ment, connect a 0.1µF capacitor from MR to GND to provide additional noise immunity. Watchdog Timer The MAX16056/MAX16058’s watchdog timer circuitry monitors the µP’s activity. If the µP does not toggle (high-to-low) the watchdog input (WDI) within the capacitor-adjustable watchdog timeout period (tWD), RESET asserts for the reset timeout period (tRP). The internal watchdog timer is cleared by: 1) any event that asserts RESET, by 2) a falling transition at WDI (that can detect pulses as short as 150ns) or by 3) a transi- tion (high-to-low or low-to-high) at WDS. While reset is asserted, the watchdog timer remains cleared and does not count. As soon as reset deasserts, the watch- dog timer resumes counting. There are two modes of watchdog operation, normal mode and extended mode. In normal mode (Figure 2), the watchdog timeout period is determined by the value of the capacitor connected between SWT and ground. In extended mode (Figure 3), the watchdog timeout period is multiplied by 128. For example, in extended mode, a 0.33µF capacitor gives a watchdog timeout period of 212s (see Table 2). To disable the watchdog timer function, connect SWT to ground. When VCC ramps above VTH + VHYST, the value of the external SWT capacitor is sampled after RESET goes high. When sampling is finished, the capacitor value is stored in the device and is used to set watchdog time- out. If RESET goes low before sampling is finished, the device interrupts sampling, and sampling is restarted when RESET goes high again. If the external SWT capacitor is less than 470pF, the sampling result sets the watchdog timeout to zero. This causes the watchdog to assert RESET continuously after sampling is finished. If a PCB manufacturing defect caused the connection to CSWT to be broken, the capacitance is very low and RESET is continuously asserted. If the external SWT capacitor is greater than 5600pF, the sampling result sets the watchdog timeout to be infinite, disabling the watchdog function. |
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