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TC1232 Arkusz danych(PDF) 9 Page - Microchip Technology |
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TC1232 Arkusz danych(HTML) 9 Page - Microchip Technology |
9 / 20 page © 2005 Microchip Technology Inc. Preliminary DS21370C-page 9 TC1232 4.3 Watchdog Timer When the ST input is not stimulated for a preset time period, the watchdog timer function forces RST and RST signals to the active state. The preset time period is determined by the TD inputs to be 150 ms with TD connected to ground, 600 ms with TD floating or 1200 ms with TD connected to VCC (typ.). The watchdog timer starts timing-out from the set time period as soon as RST and RST are inactive. If a high- to-low transition occurs on the ST input pin prior to time-out, the watchdog timer is reset and begins to time-out again. If the watchdog timer is allowed to time- out, the RST and RST signals are driven to the active state for 250 ms, minimum (Figure 4-7). The software routine that strobes ST is critical. The code must be in a section of software that is executed frequently enough so the time between toggles is less than the watchdog time-out period. One common technique controls the microprocessor I/O line from two sections of the program. The software might set the I/O line high while operating in the Foreground mode and set it low while in the Background or Interrupt modes. If both modes do not execute correctly, the watchdog timer issues reset pulses. tTD is the maximum elapsed time between ST high-to- low transitions (ST is activated by falling edges only), which will keep the watchdog timer from forcing the reset outputs active for a time of tRST. tTD is a function of the voltage at the TD pin, as tabulated below: TABLE 4-1: WATCHDOG TIMER PERIODS Figure 4-7 shows a block diagram for using the TC1232 with a PICmicro® MCU and the Watchdog input. FIGURE 4-7: Watchdog Timer. Figure 4-8 shows the expected reset output pin wave- forms depending on the period of the ST pin falling edge and the state of the TD input pin. FIGURE 4-8: Strobe Input. 4.4 Supply Monitor Noise Sensitivity The TC1232 is optimized for fast response to negative- going changes in VDD. Systems with an inordinate amount of electrical noise on VDD (such as systems using relays) may require a 0.01 µF or 0.1 µF bypass capacitor to reduce detection sensitivity. This capacitor should be installed as close to the TC1232 as possible to keep the capacitor lead length short. Condition tTD Min Typ Max TD pin = 0V 62.5 ms 150 ms 250 ms TD pin = Open 250 ms 600 ms 1000 ms TD pin = VCC 500 ms 1200 ms 2000 ms VCC TOL GND RST ST RESET +5V TC1232 TD 0.1 10 k Ω I/O PICmicro® 3-Terminal Regulator +5V µF MCU (example: MCP1700) tST ST tTD RST (when tTD ≤ tTD (min)) “H” RST (when tTD(min) ≤ tTD ≤ tTD(max)) (when tTD ≥ tTD(max)) RST RST (when tTD ≤ tTD(min)) “L” RST (when tTD(min) ≤ tTD ≤ tTD(max)) (when tTD ≥ tTD(max)) RST |
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Podobny opis - TC1232 |
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