HT56RB688_13 datasheet(42/164 Pages) HOLTEK

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Clock Source

The LCD clock source is the internal clock signal fSUB divided by 8 using an internal divider circuit. The

fSUB internal clock is supplied by either the internal 32kHz RC oscillator (LIRC) or the external

32.768kHz oscillator (LXT), 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.

SUB Clock Source

LCD Clock Frequency

Internal 32kHz oscillator (LIRC)

4kHz

External 32.768kHz oscillator (LXT)

4kHz

LCD Clock Source

LCD Driver Output

When the LCDEN bit in the LCDCTRL register is set to 1, the COM and SEG lines will be setup as

LCD driver pins to drive the LCD display.

The number of COM and SEG outputs supplied by the LCD driver and its biasing and duty selections

are dependent upon how the LCD control bits are programmed. The Bias Type is R type and the bias

resistor can be selected using the RSEL1 and RSEL0 bits in the LCDCTRL register.

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 1/8 and 1/16 and which equates to a COM

number of 8 and 16 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 LCDCTRL register. Type B offers lower frequency signals, however lower

frequencies may introduce flickering and influence display clarity.

LCD Control Register - LCDCTRL

Bit

76543210

Name

TYPE

DTYC

BIAS1

BIAS0

RSEL1

RSEL0

LCDEN

R/W

POR

000000

Bit 7

TYPE: LCD Type selection bit

0: Type A

1: Type B

Bit 6

Unimplemented, read as

²0²

Bit 5

DTYC: LCD Duty selection bit

0: 1/8 duty

1: 1/16 duty

Bit 4~3

BIAS1~BIAS0: LCD Bias selection bits

00: 1/3 bias

01: 1/4 bias

10: 1/5 bias

11: Reserved

Rev. 1.10

April 15, 2013

HT56RB688

TinyPowerTM A/D type Smart Card OTP MCU

with LCD, DAC, ISO 7816 and USB Interfaces

HT56RB688 Datasheet(PDF) 42 Page - Holtek Semiconductor Inc

HT56RB688 Datasheet(HTML) 42 Page - Holtek Semiconductor Inc

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