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ST7LITE0 データシート(PDF) 27 Page - STMicroelectronics |
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ST7LITE0 データシート(HTML) 27 Page - STMicroelectronics |
27 / 122 page ST7LITE0, ST7SUPERLITE 27/122 RESET SEQUENCE MANAGER (Cont’d) 7.4.2 Asynchronous External RESET pin The RESET pin is both an input and an open-drain output with integrated RON weak pull-up resistor. This pull-up has no fixed value but varies in ac- cordance with the input voltage. It can be pulled low by external circuitry to reset the device. See Electrical Characteristic section for more details. A RESET signal originating from an external source must have a duration of at least th(RSTL)in in order to be recognized (see Figure 16). This de- tection is asynchronous and therefore the MCU can enter reset state even in HALT mode. The RESET pin is an asynchronous signal which plays a major role in EMS performance. In a noisy environment, it is recommended to follow the guidelines mentioned in the electrical characteris- tics section. 7.4.3 External Power-On RESET If the LVD is disabled by option byte, to start up the microcontroller correctly, the user must ensure by means of an external reset circuit that the reset signal is held low until VDD is over the minimum level specified for the selected fOSC frequency. A proper reset signal for a slow rising VDD supply can generally be provided by an external RC net- work connected to the RESET pin. 7.4.4 Internal Low Voltage Detector (LVD) RESET Two different RESET sequences caused by the in- ternal LVD circuitry can be distinguished: s Power-On RESET s Voltage Drop RESET The device RESET pin acts as an output that is pulled low when VDD<VIT+ (rising edge) or VDD<VIT- (falling edge) as shown in Figure 16. The LVD filters spikes on VDD larger than tg(VDD) to avoid parasitic resets. 7.4.5 Internal Watchdog RESET The RESET sequence generated by a internal Watchdog counter overflow is shown in Figure 16. Starting from the Watchdog counter underflow, the device RESET pin acts as an output that is pulled low during at least tw(RSTL)out. Figure 16. RESET Sequences VDD RUN RESET PIN EXTERNAL WATCHDOG ACTIVE PHASE VIT+(LVD) VIT-(LVD) th(RSTL)in RUN WATCHDOG UNDERFLOW tw(RSTL)out RUN RUN RESET RESET SOURCE EXTERNAL RESET LVD RESET WATCHDOG RESET INTERNAL RESET (256 TCPU) VECTOR FETCH ACTIVE PHASE ACTIVE PHASE 1 |
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