Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
336 Low-Voltage Inhibit (LVI) MOTOROLA
Low-Voltage Inhibit (LVI)18.4 Functional Description
Figure 18-1 shows the structure of the LVI module. The LVI is enabled
out of reset. The LVI module contains a bandgap reference circuit and
comparator. The LVI power bit, LVIPWR, enables th e LVI to monitor VDD
voltage. The LVI reset bit, LVIRST, enables the LVI module to generat e
a reset when VDD falls below a voltage, VLVRX, and remains at or below
that level for nine or more consecutive CGMXCLK. VLVRX and VLVHX are
determined by the TRPSEL bit in the LVISCR (see Figure 18-2).
LVIPWR and LVIRST are in the configuration register (CONFIG). See
Section 5. Configuration Register (CONFIG).
Once an LVI reset occurs, the MCU remains in reset until VDD rises
above a voltage, VLVRX + VLVHX. VDD must be above VLVRX + VLVHX for
only one CPU cycle to bring the MCU out of reset. See 7.4.2.6
Low-Voltage Inhibit (LVI) Reset. The output of the comparator controls
the state of the LVIOUT flag in the LVI status register (LVISCR).
An LVI reset also drives the RST pin low to provide low-voltage
protection to external peripheral devices. See 22.6 DC Electrical
Characteristics (VDD = 5.0 Vdc ± 10%).
Figure 18-1. LVI Module Block Diagram
LOW VDD
LVIRST
VDD > LVItrip = 0
VDD < LVItrip = 1
LVIOUT
LVIPWR
DETECTOR
VDD
LVI RESET
FROM CONFIG
FROM CONFIG
VDD
DIGITAL FILTER
CPU CLOCK
ANLGTRIP
TRPSEL
FROM LVISCR