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MC908QY1CFQE Datasheet(PDF) 86 Page - Freescale Semiconductor, Inc |
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MC908QY1CFQE Datasheet(HTML) 86 Page - Freescale Semiconductor, Inc |
86 / 168 page Low-Voltage Inhibit (LVI) MC68HC908QY/QT Family Data Sheet, Rev. 6 86 Freescale Semiconductor VTRIPF. Setting the LVI enable in stop mode bit, LVISTOP, enables the LVI to operate in stop mode. Setting the LVI 5-V or 3-V trip point bit, LVI5OR3, enables the trip point voltage, VTRIPF, to be configured for 5-V operation. Clearing the LVI5OR3 bit enables the trip point voltage, VTRIPF, to be configured for 3-V operation. The actual trip thresholds are specified in 16.5 5-V DC Electrical Characteristics and 16.9 3-V DC Electrical Characteristics. NOTE After a power-on reset, the LVI’s default mode of operation is 3 volts. If a 5-V system is used, the user must set the LVI5OR3 bit to raise the trip point to 5-V operation. If the user requires 5-V mode and sets the LVI5OR3 bit after power-on reset while the VDD supply is not above the VTRIPR for 5-V mode, the microcontroller unit (MCU) will immediately go into reset. The next time the LVI releases the reset, the supply will be above the VTRIPR for 5-V mode. Once an LVI reset occurs, the MCU remains in reset until VDD rises above a voltage, VTRIPR, which causes the MCU to exit reset. See Chapter 13 System Integration Module (SIM) for the reset recovery sequence. The output of the comparator controls the state of the LVIOUT flag in the LVI status register (LVISR) and can be used for polling LVI operation when the LVI reset is disabled. 10.3.1 Polled LVI Operation In applications that can operate at VDD levels below the VTRIPF level, software can monitor VDD by polling the LVIOUT bit. In the configuration register, the LVIPWRD bit must be cleared to enable the LVI module, and the LVIRSTD bit must be at set to disable LVI resets. 10.3.2 Forced Reset Operation In applications that require VDD to remain above the VTRIPF level, enabling LVI resets allows the LVI module to reset the MCU when VDD falls below the VTRIPF level. In the configuration register, the LVIPWRD and LVIRSTD bits must be cleared to enable the LVI module and to enable LVI resets. 10.3.3 Voltage Hysteresis Protection Once the LVI has triggered (by having VDD fall below VTRIPF), the LVI will maintain a reset condition until VDD rises above the rising trip point voltage, VTRIPR. This prevents a condition in which the MCU is continually entering and exiting reset if VDD is approximately equal to VTRIPF. VTRIPR is greater than VTRIPF by the hysteresis voltage, VHYS. 10.3.4 LVI Trip Selection The LVI5OR3 bit in the configuration register selects whether the LVI is configured for 5-V or 3-V protection. NOTE The microcontroller is guaranteed to operate at a minimum supply voltage. The trip point (VTRIPF [5 V] or VTRIPF [3 V]) may be lower than this. See 16.5 5-V DC Electrical Characteristics and 16.9 3-V DC Electrical Characteristics for the actual trip point voltages. |
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