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G671L438T72UF Datasheet(PDF) 9 Page - Global Mixed-mode Technology Inc |
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G671L438T72UF Datasheet(HTML) 9 Page - Global Mixed-mode Technology Inc |
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9 / 13 page  Ver: 1.5 Dec 03, 2005 TEL: 886-3-5788833 http://www.gmt.com.tw 9 G670/G671 Global Mixed-mode Technology Inc. Pin Description NAME FUNCTION GND Ground (G671L/G670L) RESET Output remains low while VCC is below the reset threshold, and for at most 2ms after VCC rises above the reset threshold. RESET (G670H) RESET Output remains high while VCC is below the reset threshold, and for at most 2ms after VCC rises above the reset threshold. VCC Supply Voltage (+5V, +3.3V, +3.0V) Detailed Description A microprocessor’s (µP’s) reset input starts the µP in a known state. The G671L/G670L/G670H assert reset to prevent code-execution errors during power-up, power-down, or brownout conditions. They assert a reset signal whenever the VCC supply voltage declines below a preset threshold, keeping it asserted for at most 2ms after VCC has risen above the reset thresh- old. The G671L uses an open-drain output, and the G670L/G670H have a push-pull output stage. Connect a pull-up resistor on the G671L’s RESET output to any supply between 0 and 5.5V. Figure 1. Maximum Transient Duration Without Causing a Reset Pulse vs. Reset Com- parator Overdrive Figure 2. RESET Valid to VCC = Ground Circuit Applications Information Negative-Going VCC Transients In addition to issuing a reset to the µP during power-up, power-down, and brownout conditions, the G671L/ G670H/G670L are relatively immune to short- duration negative-going VCC transients (glitches). Figure 1 shows typical transient duration vs. reset comparator overdrive, for which the G671L/G670H/ G670L do not generate a reset pulse. The graph was generated using a negative-going pulse applied to VCC, starting 0.5V above the actual reset threshold and ending below it by the magnitude indicated (reset comparator overdrive). The graph indicates the maxi- mum pulse width a negative-going VCC transient can have without causing a reset pulse. As the magnitude of the transient increases (goes farther below the reset threshold), the maximum allowable pulse width de- creases. Typically, for the G67_ _463 and G67_ _438, a VCC transient that goes 100mV below the reset threshold and lasts 7µs or less will not cause a reset pulse. A 0.1µF bypass capacitor mounted as close as possible to the VCC pin provides additional transient immunity. Ensuring a Valid Reset Output Down to VCC = 0 When VCC falls below 1V, the G670 RESET output no longer sinks current—it becomes an open circuit. Therefore, high-impedance CMOS logic inputs con- nected to RESET can drift to undetermined voltages. This presents no problem in most applications since most µP and other circuitry is inoperative with VCC below 1V. However, in applications where RESET must be valid down to 0V, adding a pull-down resistor to RESET causes any stray leakage currents to flow to ground, holding RESET low (Figure 2). R1’s value is not critical; 100k Ω is large enough not to load RESET and small enough to pull RESET to ground. A 100k Ω pull-up resistor to VCC is also recommended for the G671L if RESET is required to remain valid for VCC < 1V. 0 100 200 300 400 500 600 1 10 100 1000 Res et Com parator Overdrive, V TH - V CC (m V ) G67_ _ 308/293/263 G67_ _ 463/438/400 V CC RESET GND G670 R1 100k V CC RESET GND G670 R1 100k |
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