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TC1270RERCTR Scheda tecnica(PDF) 6 Page - Microchip Technology |
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TC1270RERCTR Scheda tecnica(HTML) 6 Page - Microchip Technology |
6 / 14 page TC1270/TC1271 DS21381D-page 6 © 2007 Microchip Technology Inc. 4.0 APPLICATIONS INFORMATION 4.1 VCC Transient Rejection The TC1270/TC1271 provides accurate VCC monitoring and Reset timing during power-up, power-down, and brownout/sag conditions, and rejects negative-going transients (glitches) on the power supply line. Figure 4-3 shows the maximum transient duration vs. maximum negative excursion (overdrive) for glitch rejection. Any combination of duration and overdrive that lays under the curve will not generate a Reset signal. Combinations above the curve are detected as a brown-out or power-down. Transient immunity can be improved by adding a capacitor in close proximity to the VCC pin of the TC1270/TC1271. FIGURE 4-1: Maximum Transient Duration vs. Overdrive for Glitch Rejection at +25°C. 4.2 Reset Signal Integrity During Power-Down The TC1270 RESET output is valid to VCC =1.0V. Below this voltage the output becomes an “open circuit” and does not sink current. This means CMOS logic inputs to the microprocessor will be floating at an undetermined voltage. Most digital systems are completely shut down well above this voltage. However, in situations where RESET must be maintained valid to VCC = 0V, a pull-down resistor must be connected from RESET to ground to discharge stray capacitances and hold the output low (Figure 4-2). This resistor value, though not critical, should be chosen such that it does not appreciably load RESET under normal operation (100 k Ω will be suitable for most applications). Similarly, a pull-up resistor to VCC is required for the TC1271 to ensure a valid high RESET for VCC below 1.1V. FIGURE 4-2: Ensuring RESET Valid to VCC = 0V. 4.3 Processors With Bidirectional I/O Pins Some microprocessor’s (such as Motorola 68HC11) have bidirectional Reset pins. Depending on the current drive capability of the processor pin, an indeter- minate logic level may result if there is a logic conflict. This can be avoided by adding a 4.7 k Ω resistor in series with the output of the TC1270/TC1271 (Figure 4-3). If there are other components in the sys- tem which require a Reset signal, they should be buff- ered so as not to load the Reset line. If the other components are required to follow the Reset I/O of the microprocessor, the buffer should be connected as shown with the solid line. FIGURE 4-3: Interfacing to Bidirectional Reset I/O. RESET COMPARATOR OVERDRIVE, VTH - VCC (mV) 400 240 160 320 80 0 1 10 100 1000 TA = +25°C VTH Duration Overdrive VCC TC127LMJ TC127xR/S/T TC1270 VCC VCC R1 100k RESET GND TC1270 VCC RESET GND RESET GND Buffered RESET To Other System Components Buffer μP 4.7k VCC VCC |
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