S3F84B8_UM_REV 1.00 20 LOW VOLTAGE RESET20-120 LOW VOLTAGE RESET20.1 OVERVIEW OF LOW VOLTAGE RESETUsing the Smart option (3FH.7 in ROM), you can choose the reset source as internal (LVR) or external.The S3F84B8 microcontroller can be reset in four ways using: External power-on-reset External reset input pin pulled low Digital watchdog time out Low Voltage Reset circuit (LVR)During an external power-on reset, the voltage VDD is set to High level and the RESETB pin stays low level forsome time. The RESETB signal is inputted through a Schmitt trigger circuit, where it is then synchronized with theCPU clock. This brings the S3F84B8 microcontroller to a known operating status.To ensure the correct start up, you should ensure that reset signal is not released before the V DD level issufficient. This allows the MCU to operate at the chosen frequency.The RESETB pin must be held to Low level for a minimum time interval of 10us after the power supply comeswithin tolerance level. This allows time for internal CPU clock oscillation to stabilize.If a reset occurs during normal operation (with both V DD and RESETB at High level), the signal at RESETB pin isforced to Low level and the reset operation starts. All system and peripheral control registers are then set to theirdefault hardware reset values (see Figure 20-1).The MCU provides a watchdog timer function to ensure recovery from software malfunction. If the watchdog timeris not refreshed before an end-of-counter condition (overflow) is reached, the internal reset will be activated.S3F84B8 has a built-in low voltage reset (LVR) circuit that detects voltage drop of external V DD input level andprevents the MCU from malfunctioning whenever it encounters fluctuation in power level. This voltage detector isused to reset the MCU.The LVR circuit includes an analog comparator and VREF circuit. The value of detection voltage is set internallyby the hardware.The on-chip LVR circuit features static reset when supply voltage is below reference voltage value (Typical1.9/2.3/3.03.6/3.9 V). Owing to this feature, external reset circuit can be removed while keeping the applicationsafe. As long as the supply voltage is below the reference value, an internal static RESET will be triggered. TheMCU can only start when the supply voltage rises over the reference voltage.To calculate power consumption, static current of LVR circuit should be added to the CPU operating current inoperating modes such as Stop, Idle, and Normal Run.
