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MAX6444US29L Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX6444US29L Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 13 page µP Reset Circuits with Long Manual Reset Setup Period 6 _______________________________________________________________________________________ Manual Reset Input Options Unlike typical manual reset functions associated with supervisors, each device in the MAX6443–MAX6452 family includes at least one manual reset input, which must be held logic-low for an extended setup period (tMR) before the RESET output asserts. When valid manual reset input conditions/setup periods are met, the RESET output is one-shot pulse asserted low for a fixed reset timeout period (140ms min). Existing front- panel pushbutton switches (i.e., power on/off, channel up/down, or mode select) can be used to drive the manual reset inputs. The extended manual reset setup period prevents nuisance system resets during normal front-panel usage or resulting from inadvertent short- term pushbutton closure. The MAX6443/MAX6444, MAX6447/MAX6448, and MAX6451/MAX6452 include a single manual reset input with extended setup period (MR1). The MAX6445/ MAX6446 and MAX6449/MAX6450 include two manual reset inputs (MR1 and MR2) with extended setup peri- ods. For dual MR1, MR2 devices, both inputs must be held low simultaneously for the extended setup period (tMR) before the reset output is pulse asserted. The dual extended setup provides greater protection from nuisance resets. (For example, the user or service tech- nician is informed to simultaneously push both the on/off button and the channel-select button for 6.72s (L suffix) to reset the system.) The MAX6443–MAX6452 RESET output is pulse asserted once for the reset timeout period after each valid manual reset input condition. At least one manual reset input must be released (go high) and then be driven low for the extended setup period before RESET asserts again. Internal timing circuitry debounces low-to-high manual reset logic transitions, so no external circuitry is required. Figure 1 illustrates the single manual reset function of the MAX6443/MAX6444 single-voltage monitors, and Figure 2 represents the dual manual reset function of the MAX6445/MAX6446 and MAX6449/MAX6450. The MAX6447/MAX6448 and MAX6451/MAX6452 include both an extended setup period and immediate setup period manual reset inputs. A low-to-high MR2 rising edge transition immediately pulse asserts the RESET output for the reset timeout period (140ms min). If the MAX6447/MAX6448 and MAX6451/MAX6452 MR2 input senses another rising edge before the end of the 140ms timeout period (Figure 3), the internal timer clears and begins counting again. If no rising edges are detected within the 210ms timeout period, RESET deasserts. The high-to-low transition on MR2 input is internally debounced for 210ms to ensure that there are no false RESET assertions when MR2 is dri- ven from high to low (Figure 4). The MR2 input can be used for system test purposes or smart-card-detect applications (see the Applications Information section). Adjustable Input Voltage (RSTIN) The MAX6449–MAX6452 monitor the voltage on RSTIN using an adjustable reset threshold set with an external resistor voltage-divider (Figure 5). Use the following for- mula to calculate the externally monitored voltage (VMON-TH): VMON-TH = VTH-RSTIN (R1+ R2)/R2 where VMON-TH is the desired reset threshold voltage and VTH-RSTIN is the reset input threshold (0.63V). Resistors R1 and R2 can have very high values to mini- mize current consumption because of low leakage cur- rents. Set R2 to some conveniently high value (250k Ω, for example), and calculate R1 based on the desired reset threshold voltage, using the following formula: R1 = R2 (VMON-TH/VTH-RSTIN - 1) Ω RESET TIMEOUT PERIOD 210ms MR1 SETUP PERIOD tMR MR1 RESET Figure 1. MAX6443/MAX6444 Manual Reset Timing Diagram MR2 RESET MR1 210ms tMR Figure 2. MAX6445/MAX6446/MAX6449/MAX6450 Manual Reset Timing Diagram |
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