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MCP121T-315E/TO bảng dữ liệu(PDF) 13 Page - Microchip Technology |
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MCP121T-315E/TO bảng dữ liệu(HTML) 13 Page - Microchip Technology |
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13 / 28 page  2004-2014 Microchip Technology Inc. DS20001906D-page 13 MCP102/103/121/131 4.0 APPLICATION INFORMATION For many of today’s microcontroller applications, care must be taken to prevent low-power conditions that can cause many different system problems. The most common causes are brown-out conditions, where the system supply drops below the operating level momentarily. The second most common cause is when a slowly decaying power supply causes the microcontroller to begin executing instructions without sufficient voltage to sustain volatile memory (RAM), thus producing indeterminate results. Figure 4-1 shows a typical application circuit. MCP102/103/121/131 are voltage supervisor devices designed to keep a microcontroller in reset until the system voltage has reached and stabilized at the proper level for reliable system operation. These devices also operate as protection from brown-out conditions. FIGURE 4-1: Typical Application Circuit. 4.1 RST Operation The RST output pin operation determines how the device can be used and indicates when the system should be forced into reset. To accomplish this, an internal voltage reference is used to set the voltage trip point (VTRIP). Additionally, there is a hysteresis on this trip point. When the falling edge of VDD crosses this voltage threshold, the reset power-down timer (tRPD) starts. When this delay timer times out, the RST pin is forced low. When the rising edge of VDD crosses this voltage threshold, the reset power-up timer (tRPU) starts. When this delay timer times out, the RST pin is forced high, tRPU is active and there is additional system current. The actual voltage trip point (VTRIPAC) will be between the minimum trip point (VTRIPMIN) and the maximum trip point (VTRIPMAX). The hysteresis on this trip point and the delay timer (tRPU) are to remove any “jitter” that would occur on the RST pin when the device VDD is at the trip point. Figure 4-2 shows the waveform of the RST pin as determined by the VDD voltage, while Table 4-1 shows the state of the RST pin. The VTRIP specification is for falling VDD voltages. When the VDD voltage is rising, the RST will not be driven high until VDD is at VTRIP +VHYS. Once VDD has crossed the voltage trip point, there is also a minimal delay time (tRPD) before the RST pin is driven low. FIGURE 4-2: RST Operation as Determined by the VTRIP and VHYS. VDD VDD MCLR (Reset input) (Active-low) VSS PIC® Microcontroller RPU Note 1: Resistor RPU may be required with the MCP121 due to the open-drain output. Resistor RPU may not be required with the MCP131 due to the internal pull-up resistor. The MCP102 and MCP103 do not require the external pull-up resistor. 0.1 µF MCP1XX VDD RST VSS TABLE 4-1: RST PIN STATES Device State of RST Pin when: Output Driver VDD <VTRIP VDD > VTRIP + VHYS MCP102 L H Push-pull MCP103 L H Push-pull MCP121 LH (1) Open-drain (1) MCP131 LH (2) Open-drain (2) Note 1: Requires external pull-up resistor 2: Has internal pull-up resistor VDD VTRIPMAX VTRIPMIN VTRIPAC VTRIPAC VTRIPAC +VHYSAC RST 1V < 1V is outside the device specifications tRPD tRPU tRPD tRPU |
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