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ADM825 Datasheet(PDF) 10 Page - Analog Devices |
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ADM825 Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page ADM823/ADM824/ADM825 Data Sheet Rev. D | Page 10 of 12 APPLICATIONS INFORMATION WATCHDOG INPUT CURRENT To minimize the watchdog input current (and minimize overall power consumption), leave WDI low for the majority of the watchdog timeout period. When driven high, WDI can draw as much as 160 µA. Pulsing WDI low-high-low at a low duty cycle reduces the effect of the large input current. When WDI is unconnected, a window comparator disconnects the watchdog timer from the reset output circuitry so that reset is not asserted when the watchdog timer times out. NEGATIVE-GOING VCC TRANSIENTS To avoid unnecessary resets caused by fast power supply transients, the ADM823/ADM824/ADM825 are equipped with glitch rejection circuitry. The typical performance characteristic in Figure 12 plots VCC transient duration vs. the transient mag- nitude. The curves show combinations of transient magnitude and duration for which a reset is not generated for 4.63 V and 2.93 V reset threshold parts. For example, with the 2.93 V threshold, a transient that goes 100 mV below the threshold and lasts 8 µs typically does not cause a reset, but if the transient is any larger in magnitude or duration, a reset is generated. An optional 0.1 µF bypass capacitor mounted close to VCC provides additional glitch rejection. ENSURING RESET VALID TO VCC = 0 V Both active low and active high reset outputs are guaranteed to be valid for VCC as low as 1 V. However, by using an external resistor with push-pull configured reset outputs, valid outputs for VCC as low as 0 V are possible. For an active low reset output, a resistor connected between RESET and ground pulls the output low when it is unable to sink current. For an active high reset output, a resistor connected between RESET and VCC pulls the output high when it is unable to source current. A large resist- ance such as 100 kΩ should be used so that the reset output is not overloaded when VCC is above 1 V. ADM823/ ADM824/ ADM825 VCC RESET 100kΩ ADM824/ ADM825 VCC RESET 100kΩ Figure 17. Ensuring Reset Valid to VCC = 0 V WATCHDOG SOFTWARE CONSIDERATIONS In implementing the microprocessor watchdog strobe code, quickly switching WDI low-to-high and then high-to-low (minimizing WDI high time) is desirable for current consumption reasons. However, a more effective way of using the watchdog function can be considered. A low-high-low WDI pulse within a given subroutine prevents the watchdog timing out. However, if the subroutine becomes stuck in an infinite loop, the watchdog cannot detect this cond- ition because the subroutine continues to toggle WDI. A more effective coding scheme for detecting this error involves using a slightly longer watchdog timeout. In the program that calls the subroutine, WDI is set high (see Figure 18). The subroutine sets WDI low when it is called. If the program executes without error, WDI is toggled high and low with every loop of the program. If the subroutine enters an infinite loop, WDI is kept low, the watchdog times out, and the microprocessor is reset. START SET WDI HIGH PROGRAM CODE SUBROUTINE SET WDI LOW RETURN INFINITE LOOP: WATCHDOG TIMES OUT RESET Figure 18. Watchdog Flow Diagram RESET RESET WDI I/O MR ADM823 VCC MICROPROCESSOR Figure 19. Typical Application Circuit |
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