MC68HC908GR8 Datasheet, PDF (128/286 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Low-Voltage Inhibit (LVI)
LVISTOP, LVIPWRD, LVI5OR3, and LVIRSTD are in the configuration register (CONFIG). See Chapter
8 Configuration Register (CONFIG) for details of the LVI’s configuration bits. Once an LVI reset occurs,
the MCU remains in reset until V_DD rises above a voltage, V_TRIPR, which causes the MCU to exit reset.
See 19.3.2.5 Low-Voltage Inhibit (LVI) Reset for details of the interaction between the SIM and the LVI.
The output of the comparator controls the state of the LVIOUT flag in the LVI status register (LVISR).
An LVI reset also drives the RST pin low to provide low-voltage protection to external peripheral devices.
V_DD
LVIPWRD
FROM CONFIG
STOP INSTRUCTION
FROM CONFIG
LVIRSTD
LVISTOP
FROM CONFIG
LOW V_DD
DETECTOR
V_DD > LVI_Trip = 0
V_DD ≤ LVI_Trip = 1
LVI5OR3
FROM CONFIG
LVIOUT
Figure 14-1. LVI Module Block Diagram
LVI RESET
Addr. Register Name
Bit 7
6
5
4
3
2
1
Bit 0
Read: LVIOUT
0
0
0
0
0
0
0
$FE0C
LVI Status Register
(LVISR)
Write:
Reset: 0
0
0
0
0
0
0
0
= Unimplemented
Figure 14-2. LVI I/O Register Summary
14.3.1 Polled LVI Operation
In applications that can operate at V_DD levels below the V_TRIPF level, software can monitor V_DD by polling
the LVIOUT bit. In the configuration register, the LVIPWRD bit must be 0 to enable the LVI module, and
the LVIRSTD bit must be 1 to disable LVI resets.
14.3.2 Forced Reset Operation
In applications that require V_DD to remain above the V_TRIPF level, enabling LVI resets allows the LVI
module to reset the MCU when V_DD falls below the V_TRIPF level. In the configuration register, the
LVIPWRD and LVIRSTD bits must be cleared to enable the LVI module and to enable LVI resets.
14.3.3 Voltage Hysteresis Protection
Once the LVI has triggered (by having V_DD fall below V_TRIPF), the LVI will maintain a reset condition until
V_DD rises above the rising trip point voltage, V_TRIPR. This prevents a condition in which the MCU is
continually entering and exiting reset if V_DD is approximately equal to V_TRIPF. V_TRIPR is greater than
V_TRIPF by the hysteresis voltage, V_HYS.
MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7
128
Freescale Semiconductor