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Rev. 1.20
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HT69F30A/HT69F40A/HT69F50A
TinyPower
TM
I/O Flash 8-Bit MCU with LCD & EEPROM
HT69F30A/HT69F40A/HT69F50A
TinyPower
TM
I/O Flash 8-Bit MCU with LCD & EEPROM
LCD Reset Function
The LCD has an internal reset function that is an OR function of the inverted LCDEN bit in the
LCDC register and the Sleep function. When the LCDEN bit is set to 1 to enable the LCD driver
function before the device enters the SLEEP mode, the LCD function will be reset after the device
enters the SLEEP mode. Clearing the LCDEN bit to zero will also reset the LCD function.
LCDEN
SLEEP Mode
Reset LCD
0
�ff
√
0
�n
√
1
�ff
x
1
�n
√
LCD Reset Function
Clock Source
The LCD clock source is the internal clock signal, f
SUB
, divided by 8, using an internal divider
circuit. The f
SUB
internal clock is supplied by either the LIRC or LXT oscillator, the choice of which
is determined by a configuration option. For proper LCD operation, this arrangement is provided to
generate an ideal LCD clock source frequency of 4kHz.
f
SUB
Clock Source
LCD Clock Frequency
LIRC
4kHz
LXT
4kHz
LCD Clock Source
LCD Driver Output
The number of COM and SEG outputs supplied by the LCD driver, as well as its biasing and duty
selections, are dependent upon how the LCD control bits are programmed. The Bias Type, whether
C or R type is selected via a configuration option.
The nature of Liquid Crystal Displays require that only AC voltages can be applied to their pixels
as the application of DC voltages to LCD pixels may cause permanent damage. For this reason the
relative contrast of an LCD display is controlled by the actual RMS voltage applied to each pixel,
which is equal to the RMS value of the voltage on the COM pin minus the voltage applied to the
SEG pin. This differential RMS voltage must be greater than the LCD saturation voltage for the
pixel to be on and less than the threshold voltage for the pixel to be off.
The requirement to limit the DC voltage to zero and to control as many pixels as possible with
a minimum number of connections, requires that both a time and amplitude signal is generated
and applied to the application LCD. These time and amplitude varying signals are automatically
generated by the LCD driver circuits in the microcontroller. What is known as the duty determines
the number of common lines used, which are also known as backplanes or COMs. The duty, which
is chosen by a control bit to have a value of 1/3 or 1/4 and which equates to a COM number of 3
or 4 respectively, therefore defines the number of time divisions within each LCD signal frame.
Two types of signal generation are also provided, known as Type A and Type B, the required type
is selected via the TYPE bit in the LCDC register. Type B offers lower frequency signals, however
lower frequencies may introduce flickering and influence display clarity.