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MAX820MCPE Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX820MCPE Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 16 page Microprocessor and Non-Volatile Memory Supervisory Circuits 8 _______________________________________________________________________________________ Detailed Description Manual-Reset Input Many µP-based products require manual-reset capabil- ity, allowing the operator to initiate a reset. The manu- al/external-reset input (MR) can connect directly to a switch without an external pull-up resistor or debounc- ing network. MR internally connects to a 1.30V com- parator, and has a high-impedance pull-up to VCC, as shown in Figure 1. The propagation delay from assert- ing MR to reset asserted is typically 12µs. Pulsing MR low for a minimum of 25µs asserts the reset function (see Reset Function section). The reset output remains active as long as MR is held low, and the reset timeout period begins after MR returns high (Figure 2). To pro- vide extra noise immunity in high-noise environments, pull MR up to VCC with a 100kΩ resistor. Use MR as either a digital logic input or as a second low- line comparator. Normal TTL/CMOS levels can be wire-OR connected via pull-down diodes (Figure 3), and open-drain/collector outputs can be wire-ORed directly. Monitoring the Regulated Supply The MAX792/MAX820 offer two modes for monitoring the regulated supply and providing reset and non- maskable interrupt (NMI) signals to the µP: internal threshold mode uses the factory preset low-line and reset thresholds, and external programming mode allows the low-line and reset thresholds to be pro- grammed externally using a resistor voltage divider (Figure 4). Internal Threshold Mode Connecting the reset-input/internal-mode select pin (RESET IN/INT) to ground selects internal threshold mode (Figure 4a). In this mode, the low-line and reset thresholds are factory preset by an internal voltage divider (Figure 1) to the threshold voltages specified in the Electrical Characteristics (Reset Threshold Voltage and Low-Line Threshold Voltage). Connect the low-line output (LOWLINE) to the µP NMI pin, and connect the active-high reset output (RESET) or active-low reset output (RESET) to the µP reset input pin. Additionally, the low-line input/reference-output pin (LLIN/REFOUT) connects to the internal 1.30V refer- ence in internal threshold mode. Buffer LLIN/REFOUT with a high-impedance buffer to use it with external circuitry. In this mode, when VCC is falling, LOWLINE is guaranteed to be asserted prior to reset assertion. External Programming Mode Connecting RESET IN/INT to a voltage above 600mV selects external programming mode. In this mode, the low-line and reset comparators disconnect from the inter- nal voltage divider and connect to LLIN/REFOUT and RESET IN/INT, respectively (Figure 1). This mode allows flexibility in determining where in the operating voltage range the NMI and reset are generated. Set the low-line and reset thresholds with an external resistor divider, as in Figure 4b or Figure 4c. RESET typically remains valid for VCC down to 2.5V; RESET is guaranteed to be valid with VCC down to 1V. Calculate the values for the resistor voltage divider in Figure 4b using the following equations: 1) R3 = (1.30 x VCC MAX)/(VLOW LINE x IMAX) 2) R2 = [(1.30 x VCC MAX)/(VRESET x IMAX)] - R3 3) R1 = (VCC MAX/IMAX) - (R2 + R3). First choose the desired maximum current through the voltage divider (IMAX) when VCC is at its highest (VCC MAX). There are two things to consider here. First, IMAX contributes to the overall supply current for the circuit, so you would generally make it as small as possible. Second, IMAX cannot be too small or leakage currents will adversely affect the programmed threshold voltages; 5µA is often appropriate. Determine R3 after you have chosen IMAX. Use the value for R3 to determine R2, then use both R2 and R3 to determine R1. For example, to program a 4.75V low-line threshold and a 4.4V reset threshold, first choose IMAX to be 5µA when VCC = 5.5V and substitute into equation 1. R3 = (1.30 x 5.5)/(4.75 x 5E-6) = 301.05k Ω. 301k Ω is the nearest standard 0.1% value. Substitute into equation 2: R2 = [(1.30 x 5.5)/(4.4 x 5E-6)] - 301k Ω = 23.95kΩ. The nearest 0.1% resistor value is 23.7k Ω. Finally, sub- stitute into equation 3: R1 = (5.5/5E-6) - (23.7k Ω + 301kΩ) = 775kΩ. The nearest 0.1% value resistor is 787k Ω. Determine the actual low-line threshold by rearranging equation 1 and plugging in the standard resistor values. The actual low- line threshold is 4.75V and the actual reset threshold is 4.40V. An additional resistor allows the MAX792/MAX820 to monitor the unregulated supply and provide an NMI before the regulated supply begins to fall (Figure 4c). Both of these thresholds will vary from circuit to circuit with resistor tolerance, reference variation, and compara- tor offset variation. The initial thresholds for each circuit will also vary with temperature due to reference and off- set drift. For highest accuracy, use the MAX820. |
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