
Low-Voltage Inhibit (LVI)
MC68HC908GR8 MC68HC908GR4 Data Sheet, Rev. 7
128
Freescale Semiconductor
LVISTOP, LVIPWRD, LVI5OR3, and LVIRSTD are in the configuration register (CONFIG). See
Chapterthe MCU remains in reset until VDD rises above a voltage, VTRIPR, which causes the MCU to exit reset.
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.
Figure 14-1. LVI Module Block Diagram
14.3.1 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 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 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.
14.3.3 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.
Addr.
Register Name
Bit 7
654321
Bit 0
$FE0C
LVI Status Register
(LVISR)
Read:
LVIOUT
0000000
Write:
Reset:
00000000
= Unimplemented
Figure 14-2. LVI I/O Register Summary
LOW VDD
DETECTOR
LVIPWRD
STOP INSTRUCTION
LVISTOP
LVI RESET
LVIOUT
VDD > LVITrip = 0
VDD ≤ LVITrip = 1
FROM CONFIG
VDD
FROM CONFIG
LVIRSTD
LVI5OR3
FROM CONFIG