HT49CA1 View Datasheet(PDF) - Holtek Semiconductor
Part Name
Description
MFG CO.
'HT49CA1' PDF : 58 Pages View PDF
HT49RA1/HT49CA1
LCD Voltage Source and Biasing
The time and amplitude varying LCD signals generated
by the microcontroller require the generation of several
voltage levels for their operation. The number of voltage
levels used by the signal depends upon device and the
chosen bias configuration options.
LCD Biasing
The device has a configuration option to select either
1/2 or 1/3 bias. For the 1/2 bias configuration option,
three voltage levels VSS, VA and VB are utilised. VB is
generated internally by the microcontroller and will have
a value equal to VLCD/2. For the 1/3 bias option, four
voltage levels VSS, VA, VB and VC are utilised. An ex-
ternal LCD voltage source is also provided on pin VLCD
to generate these voltages. As the C type bias option
uses a charge pump circuit, higher voltages than what is
provided externally on VLCD can be generated. This
feature is useful in applications where the
microcontroller supply voltage is less than the supply
voltage required by the LCD.
As the LCD driver has a C type bias, a charge-pump ca-
pacitor between pins C1 and C2 and filter capacitors on
pins V1 and V2 are required to generate the necessary
voltage levels.
Programming Considerations
Certain precautions must be taken when programming
the LCD. One of these is to ensure that the LCD memory
is properly initialized after the microcontroller is pow-
ered on. Like the General Purpose Data Memory, the
contents of the LCD memory are in an unknown condi-
tion after power-on. As the contents of the LCD memory
will be mapped into the actual LCD, it is important to ini-
tialize this memory area into a known condition soon af-
ter applying power to obtain a proper display pattern.
Consideration must also be given to the capacitive load
of the actual LCD used in the application. As the load
presented to the microcontroller by LCD pixels can be
generally modeled as mainly capacitive in nature, it is
important that this is not excessive, a point that is partic-
ularly true in the case of the COM lines which may be
connected to many LCD pixels. The accompanying dia-
gram depicts the equivalent circuit of the LCD.
SEG 0 SEG 1 SEG 2
SEG n
COM 0
COM 1
COM 2
COM 3
LCD Panel Equivalent Circuit
Setting the correct frequency of the LCD clock is an-
other factor which must be taken into account in user ap-
plications. To have the LCDs operate at their best frame
frequency, which is normally between 25Hz and 250Hz,
it is important to select an appropriate LCD clock fre-
quency configuration option. The correct option should
be chosen to ensure that an LCD clock frequency as
close to 4kHz as possible is achieved. With such a fre-
quency chosen, the microcontroller internal LCD driver
circuits will ensure that the appropriate LCD driving sig-
nals are generated to obtain a suitable LCD frame fre-
quency.
Note that as the LCD driver will consume a certain
amount of power it can be disabled using the LCDEN bit
in the LCDC register. In battery applications where
power consumption is an important consideration to
prolong battery life, this bit should be used to power
down the LCD circuitry to conserve power.
VA
(= V L C D ´ 1 .5 )
VB
(= V L C D )
C h a rg e
P um p
V LC D
C1
C2
V1
VC
V2
(= V L C D ´ 0 .5 )
C ty p e 1 /3 B ia s
LC D
P o w e r S u p p ly
0 .1 m F
0 .1 m F
0 .1 m F
V LC D
VA
C1
(= V L C D )
C h a rg e
C2
P um p
V1
VB
(= V L C D ´ 0 .5 )
V2
C ty p e 1 /2 B ia s
C Type Bias Voltage Levels
LC D
P o w e r S u p p ly
0 .1 m F
0 .1 m F
0 .1 m F
Rev. 1.10
23
March 30, 2014
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