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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
W
atchdog T
imer
Low V
oltage Detector – L
VD
23
Low Voltage Detector – LVD
Each device has a Low Voltage Detector function, also known as LVD. This enables the device
to monitor the power supply voltage, V
DD
, and provide an interrupt should it fall below a certain
level. This function may be especially useful in battery applications where the supply voltage will
gradually reduce as the battery ages, as it allows a battery low early warning signal to be generated.
The LVD function can also generate an interrupt signal if required.
LVD Register
The Low Voltage Detector function is controlled using a single register with the name LVDCR.
Three bits in this register, LVDS2~LVDS0, are used to select one of eight fixed voltages below
which a low voltage condition will be determined. The LVDEN bit is used to control the overall
on/off function of the low voltage detector. Setting the bit high will enable the low voltage detector.
Clearing the bit to zero will switch off the internal low voltage detector circuits. As the low voltage
detector will consume a certain amount of power, it may be desirable to switch off the circuit when
not in use, an important consideration in power sensitive battery powered applications.
LVDCR Register
SFR Address: EBh
Bit
7
6
5
4
3
2
1
0
Name
LVDEN
—
—
—
—
LVDS�
LVDS1
LVDS0
R/W
R/W
—
—
—
—
R/W
R/W
R/W
POR
0
—
—
—
—
0
0
0
Bit 7
LVDEN:
LVD Function Control
0: Disable
1: Enable
Bit 6~3
Unimplemented, read as "0"
Bit 2~0
LVDS2~LVDS0:
Select LVD Voltage
000: 2.0V
001: 2.2V
010: 2.4V
011: 2.7V
100: 3.0V
101: 3.3V
110: 3.6V
111: 4.2V
LVD Operation
The Low Voltage Detector function operates by comparing the power supply voltage, V
DD
, with
a pre-specified voltage level stored in the LVDC
R register. This has a range of between 2.0V and
4.2V. When the power supply voltage, V
DD
, falls below this pre-determined value and if the LVD
interrupt function is enabled, the LVD interrupt will take place and the interrupt request flag,
LVDF, in the IRCON2 register, will be set high. The LVDF bit will be cleared to low by hardware
automatically. The LVD interrupt can cause the device to wake-up from the IDLE Mode. If the
Low Voltage Detector wake up function is not required then the LVDF flag should be first set high
and disable the LVD interrupt function before the device enters the IDLE Mode. When the device
is powered down the low voltage detector will be disabled to reduce the power consumption.