MC9S08AW16CFUE Datasheet, PDF (71/324 Pages) Freescale Semiconductor, Inc – MC9S08AW60 Features

Chapter 5 Resets, Interrupts, and System Configuration
5.6 Low-Voltage Detect (LVD) System
The MC9S08AW60 Series includes a system to protect against low voltage conditions in order to protect
memory contents and control MCU system states during supply voltage variations. The system is
comprised of a power-on reset (POR) circuit and an LVD circuit with a user selectable trip voltage, either
high (V_LVDH) or low (V_LVDL). The LVD circuit is enabled when LVDE in SPMSC1 is high and the trip
voltage is selected by LVDV in SPMSC2. The LVD is disabled upon entering any of the stop modes unless
the LVDSE bit is set. If LVDSE and LVDE are both set, then the MCU cannot enter stop2, and the current
consumption in stop3 with the LVD enabled will be greater.
5.6.1 Power-On Reset Operation
When power is initially applied to the MCU, or when the supply voltage drops below the V_POR level, the
POR circuit will cause a reset condition. As the supply voltage rises, the LVD circuit will hold the chip in
reset until the supply has risen above the V_LVDL level. Both the POR bit and the LVD bit in SRS are set
following a POR.
5.6.2 LVD Reset Operation
The LVD can be configured to generate a reset upon detection of a low voltage condition by setting
LVDRE to 1. After an LVD reset has occurred, the LVD system will hold the MCU in reset until the supply
voltage has risen above the level determined by LVDV. The LVD bit in the SRS register is set following
either an LVD reset or POR.
5.6.3 LVD Interrupt Operation
When a low voltage condition is detected and the LVD circuit is configured for interrupt operation (LVDE
set, LVDIE set, and LVDRE clear), then LVDF will be set and an LVD interrupt will occur.
5.6.4 Low-Voltage Warning (LVW)
The LVD system has a low voltage warning flag to indicate to the user that the supply voltage is
approaching, the LVD voltage. The LVW does not have an interrupt associated with it. There are two user
selectable trip voltages for the LVW, one high (V_LVWH) and one low (V_LVWL). The trip voltage is selected
by LVWV in SPMSC2. Setting the LVW trip voltage equal to the LVD trip voltage is not recommended.
Typical use of the LVW would be to select V_LVWH and V_LVDL.
5.7 Real-Time Interrupt (RTI)
The real-time interrupt function can be used to generate periodic interrupts. The RTI can accept two
sources of clocks, the 1-kHz internal clock or an external clock if available. The 1-kHz internal clock
source is completely independent of any bus clock source and is used only by the RTI module and, on some
MCUs, the COP watchdog. To use an external clock source, it must be available and active. The RTICLKS
bit in SRTISC is used to select the RTI clock source.
MC9S08AW60 Data Sheet, Rev 2
Freescale Semiconductor
71