HT8 MCU Integrated LCD Application Guideline - holtek.com · HT8 MCU Integrated LCD Application...

HT8 MCU Integrated LCD Application Guideline

AN0410E V1.00 1 / 13 December 11, 2016


D/N: AN0410E

Introduction The Holtek LCD type MCUs provide four LCD driving schemes including the R type, C

type, SCOM type as well as SCOM and SSEG type, each of which has its own features.

This guideline will firstly introduce the LCD driving principle and then gradually introduce

the usage, advantages and disadvantages of each driving type. With the help of this

guideline, users will have a deeper understanding of the Holtek LCD type MCUs and use

them more proficiently.

Functional Description

LCD Display Basic Principle Description

The Liquid Crystal Display, also known as LCD, is a technology that uses of the physical

structure and optical characteristics of the liquid crystal molecules for display.

Features of liquid crystal molecules:

Liquid crystal molecules are rod-like structure macromolecules in a state between solid and liquid.

In their natural form, they are characterised by optical anisotropy. In an electric (magnetic) field, they are characterised by isotropy.

The following section uses the basic structure of the direct-view style simple multiplexer

TN/STN LCD panel to describe the LCD basic display principle, as shown below.

Polarizer PolarizerGlass SubstrateGlass SubstrateLiquid Crystals Coating

Twisted Structure

ACNatural Light

(a) No voltage is applied between the glass substrates


AN0410E V1.00 2 / 13 December 11, 2016

Polarizer PolarizerGlass SubstrateGlass SubstrateLiquid Crystals Coating

Twisted Structure

ACNatural Light

(b) A voltage is applied between the glass substrates

LCD Basic Display Principle

A complete LCD panel is composed of the upper and lower glass substrates and

polarizers. Between the upper and lower glasses, the liquid crystal molecules are

regularly coated in a twisted structure. The electrodes are etched on the inside faces of

the upper and lower glass substrates using an ITO metal compound. As shown in the

figures, the liquid crystal molecules which are arranged in a twisted structure have the

optical activity. The polarization angles of the upper and lower polarisers are mutually

perpendicular. When the voltage between the upper and lower glasses is zero, only the

natural light with the same direction as the first polariser can pass through it and reach the

twisted liquid crystals coating. Due to the optical activity, the entered light is rotated by 90

degrees in direction and illuminates on the other polariser. Since the polarisation angles

of the upper and lower polarisers are mutually perpendicular, the entered light can

completely pass through the polariser on the other side and reaches the viewer’s eyes, in

which case, the LCD appears white. When there is an AC voltage applied between the

two glasses, on the effect of an electric (magnetic) field the liquid crystals structure is

changed from the twisted form to the synthetic form, which will have no rotation effect on

the entered light direction. Due to the aforementioned feature of the two polarisers, the

entered light is blocked from passing through the other polariser to reach the viewer’s

eyes, in which case the LCD appears black. The LCD white (on) and black (off) states are

implemented by applying different AC voltages between the upper and lower glass

substrates.

There are several critical parameters regarding the liquid crystal analog driving voltage.

AC voltage: The nature of liquid crystals determines that they can only be driven by an AC voltage. The long existence of a DC voltage applied to the liquid crystals will influence their electrical and chemical characteristics and may cause permanent damages such as blurred display and reduced lifetime, etc.

Scanning frequency: The frequency of the AC voltage applied to drive the liquid crystals is generally in the range of 60~100Hz. The actual frequency is determined by the LCD panel area and design. If the frequency is too high, it will lead to an increase in driver power consumption. If the frequency is too low, it will result in display flashing, which will also occur when there is a multiple relationship between the scanning frequency and light source frequency.


AN0410E V1.00 3 / 13 December 11, 2016

Frame Schematic Diagram

The liquid crystal molecule is a voltage integral type material. Its distortion degree, or

transmittance, is only related to the voltage effective value between the two glasses and

unrelated to the charging wave. The voltage effective value, which is expressed as

V(RMS), equals to the RMS value of the voltage on the COM pin minus the voltage on the

SEG pin.

V(RMS) = 2

0

1 ])t(V[T

T

∫ dt

The cut-off point of the LCD black (un-transparent) and white (transparent) display RMS

voltages, is called as turn on voltage, Vth. When the RMS voltage exceeds Vth, the

rotation angle of the twisted structure increases resulting in a significant change in light

transmittance and in turn a significant rise in transparent degree, whereas the transparent

degree decreases greatly. The relationship between the light transmittance and the RMS

voltage value is shown below.

Light Transmittance vs. AC Voltage RMS Value

The LCD driver in a LCD type microcontroller is controlled by the system to generate the

required driving waves according to the user-defined display image. The driving signals

are connected to the LCD panel to turn on the associated pixels for the desired display

result.


AN0410E V1.00 4 / 13 December 11, 2016

Duty

This parameter is also known as Duty counts or COM counts. The STN/TN type LCDs

usually use the dynamic time-sharing scan mode, in which case the ratio of the valid

enable time of each COM to the whole scanning cycle, namely duty, is fixed and equals to

1/COM counts.

Bias

The driving waves applied to the LCD SEG and COM driver are analog signals. The ratio

of each voltage level to the maximum LCD output voltage is indicated as bias. Usually,

the bias indicates the ratio of the lowest voltage level to the maximum output voltage.

Take a look at the following example with a configuration of 1/4 duty and 1/3 bias.

LCD Driver Waveform

The above figure shows the waveform with a configuration of 1/4 duty and 1/3 bias. There

are four COMs so the ratio of the valid enable time of each COM to the entire scanning

cycle is 1/4. There are three analog driving voltage levels, V3 is the maximum output

voltage level, V2 and V1 are the medium voltage levels and V1/V3 equals to 1/3. That is

how the configuration of 1/4 duty and 1/3 bias is implemented.

Generally, there is a certain relationship between the bias and duty parameters. The

larger the duty is the shorter the scanning time of each COM will be. To achieve the same

display brightness and display contrast, the Von value should be increased, and the

difference between the select level and unselect level should be greater which means the

bias value should be larger. The relationship between duty and bias is expressed as the

following formula.

Bias = 1 / ( Duty +1)


AN0410E V1.00 5 / 13 December 11, 2016

R Type and C Type LCD For R type biasing its bias voltage is supplied by internal bias resistors. For C type biasing

its bias voltage is supplied by an internal charge pump. Take the BS67Fxx series as an

example.

Device Duty Bias Bias Type Wave Type

BS67F340 BS67F350 BS67F360

1/4 1/3 R or C A or B

LCD Selections

QT: Quick charging time determined by QCT [2:0]

QTCOMn

PLCD

VA control

VAQuick charging control

QCT

Note: When the R type LCD is disabled, the DC path will be switched.

VIN

VB

VC

LCDEN

R Type Bias Configuration − 1/3 Bias


AN0410E V1.00 6 / 13 December 11, 2016

Note: The pin VMAX must be connected to the maximum voltage to prevent from the pad leakage.

Power Supply from pin V2

Charge Pump

VA=V1=3*VIN

VB=PLCD=2*VIN

VC=V2=VIN

0.1uF

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD

VIN

VDD or V1

Charge Pump

VA=V1=VIN

VB=PLCD=2/3*VIN

VC=V2=1/3*VIN

0.1uF

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD

Power Supply from pin V1

VIN

VDD or V1

Charge Pump

VA=V1=3/2*VIN

VB=PLCD=VIN

VC=V2=1/2*VIN

0.1uF

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD VIN

Power Supply from pin PLCD

VDD or V1

C Type Bias External Power Supply Configuration − 1/3 Bias

Note: The pin VMAX must be connected to the maximum voltage to prevent from the pad leakage.

Power Supply from VA

Charge Pump

VA=V1=VIN

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD

V1

0.1uF

VDDVIN

VB=PLCD=2/3*VIN

VC=V2=1/3*VIN

0.1uF

Charge Pump

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD

Power Supply from VB

V1

VA=V1=3/2*VIN

VDDVIN VB=PLCD=VIN

VC=V2=1/2*VIN

0.1uF

0.1uF

Power Supply from VC

Charge Pump

0.1uF

0.1uF

C1

C2

V1

V2

VMAX

PLCD

VDD or V1

VA=V1=3*VIN

VREFINVIN

VB=PLCD=2*VIN

VC=V2=VIN

0.1uF

0.1uF

C Type Bias Internal Power Supply Configuration − 1/3 Bias

Power Consumption

The R type biasing is quite suitable for LCD screens with a larger load current but

generates higher power consumption. The C type biasing generates lower power

consumption but provides a smaller current to the external loads. The table below

presents the LCD DC electrical characteristics of the BS67Fxx series for reference.

Symbol Parameter Test Conditions Min. Typ. Max. Unit VDD Conditions

ILCD

Additional Current Consumption for LCD Enable (R type)

3V RT=1170KΩ, VA=VPLCD=VDD, 1/3 bias, no load

12.5 μA

5V 25 μA


25 μA

5V 50 μA


100 μA

5V 160 μA

Additional Current Consumption for LCD Enable (C type)

3V No load, C type

2 μA

5V 2.6 μA

ILCDOL LCD Common and Segment Sink Current

3V VOL = 0.1VDD 210 420 μA 5V 350 700 μA

ILCDOH LCD Common and Segment Source Current

3V VOH = 0.9VDD -80 -160 μA 5V -180 -360 μA

As the table shows, when the VDD is 3V, the smallest LCD driving current is 12.5µA for the

R type and 2µA for the C type.


AN0410E V1.00 7 / 13 December 11, 2016

LCD Memory

An area of Data Memory is especially reserved for use for the LCD display data. This data

area is known as the LCD Memory. Any data written here will be automatically read by the

internal display driver circuits, which will in turn automatically generate the necessary

LCD driving signals. Therefore any data written into this Memory will be immediately

reflected into the actual display connected to the microcontroller. As the LCD Memory

addresses overlap those of the General Purpose Data Memory, it is stored in its own

independent Sector 4 area. The Data Memory sector to be used is chosen by using the

Memory Pointer high byte register, which is a special function register in the Data Memory,

with the name, MP1H or MP2H. To access the LCD Memory therefore requires first that

Sector 4 is selected by writing a value of 04H to the MP1H or MP2H register. After this,

the memory can then be accessed by using indirect addressing through the use of

Memory Pointer low byte, MP1L or MP2L. With Sector 4 selected, then using MP1L or

MP2L to read or write to the memory area, starting with address “00H”, will result in

operations to the LCD Memory. Directly addressing the LCD Display Memory can be

applicable using the extended instructions for the full range address access.

The accompanying LCD Memory Map diagram shows how the internal LCD Memory is

mapped to the Segments and Commons of the display for one device of this series.

00H

01H

02H

03H

1CH

1DH

1EH

1FH

SEG 0

SEG 1

SEG 2

SEG 3

SEG 31

SEG 30

SEG 29

SEG 28

b3 b2 b1 b0

CO

M 0

CO

M 1

CO

M 2

CO

M 3

32 SEG x 4 COM

BS67F350 LCD Memory Map


AN0410E V1.00 8 / 13 December 11, 2016

LCD Registers

There are control registers, named as LCDC0 and LCDC1, in the Data Memory which are

used to control the various setup features of the LCD Driver. Various bits in these

registers control functions such as LCD wave type, bias type, supply power selection,

total bias resistor selection together with the overall LCD enable and disable control.

The LCDEN bit in the LCDC0 register, which provides the overall LCD enable/disable

function, will only be effective when the device is in the NOAMRL, SLOW or IDLE Mode. If

the device is in the SLEEP Mode then the display will always be disabled. Bits,

RSEL2~RSEL0, in the LCDC0 register select the internal total bias resistors to supply the

LCD panel with the proper R type bias current. A choice to best match the LCD panel

used in the application can be selected also to minimise bias current. The TYPE bit in the

LCDC0 register is used to select whether Type A or Type B LCD waveform signals are

used. The RCT bit in the same register is used to select whether R Type or C Type LCD

bias is used. The LCDP1 and LCDP0 bits are used to select that the LCD supply power

comes from either the external pin or internal power supply for C type bias application.

The PLCD3~PLCD0 bits in the LCDC1 register are used to select the VA voltage for R

type bias circuitry. The QCT2~QCT0 bits in the same register are used to determine the

quick charge time period.

Register Name

Bit

7 6 5 4 3 2 1 0

LCDC0 TYPE RCT LCDP1 LCDP0 RSEL2 RSEL1 RSEL0 LCDEN

LCDC1 QCT2 QCT1 QCT0 PLCD3 PLCD2 PLCD1 PLCD0

LCD Voltage Source and Biasing

The time and amplitude varying signals generated by the LCD driver function require the

generation of several voltage levels for their operation. The devices can have either R

type or C type biasing selected via a software control bit named RCT. Selecting the C

type biasing will enable an internal charge pump circuitry.

R Type Biasing

For R type biasing an external LCD voltage source must be supplied on pin PLCD to

generate the internal biasing voltages. This could be the microcontroller power supply

VDD or some other voltage source equal to or less than VDD. For the R type 1/3 bias

scheme, four voltage levels VSS, VA, VB and VC are utilised. The voltage VA is selected by

the PLCD3~PLCD0 bits to be equal to a specific ratio of VPLCD varying from 8/16 VPLCD to

VPLCD. The voltage VB is equal to VA×2/3 while the voltage VC is equal to VA×1/3.

Different values of internal bias resistors can be selected using the RSEL2~RESEL0 bits

in the LCDC0 register. This along with the voltage on pin PLCD will determine the bias

current. The VMAX pin should be connected to the VDD pin since the available maximum

voltage applied to the PLCD pin is equal to VDD. Note that no external capacitors or

resistors are required to be connected if R type biasing is used.


AN0410E V1.00 9 / 13 December 11, 2016

Condition VMAX Connection

VDD ≥ VPLCD Connect VMAX to VDD

VDD < VPLCD Forbidden condition

R Type Bias VMAX Pin Connection

C Type Biasing

For C type biasing the LCD voltage source can be supplied on the external pin or derived

from the internal voltage source, which is selected using the LCDP1 and LCDP0 bits in

the LCDC0 register. The LCD voltage source which is used to generate the required

biasing voltages, is supplied on the external pin PLCD, V1 or V2 when the LCDP1 and

LCDP0 bits equal to 00B, or is derived from the internal voltage source when these bits

equal to 01B, 10B or 11B. The C type biasing scheme using an internal charge pump

circuit can generate voltages higher than what is supplied on PLCD. This feature is useful

in applications where the microcontroller supply voltage is less than the supply voltage

required by the LCD. Additional charge pump capacitors must also be connected

between pins C1 and C2 to generate the necessary voltage levels.

LCD Power Supply VA Voltage VB Voltage VC Voltage

External Power Supply

VIN on V1 VIN 2/3 × VIN 1/3 × VIN VIN on PLCD 3/2 × VIN VIN 1/2 × VIN

VIN on V2 3 × VIN 2 × VIN VIN

Internal Power Supply

VDD on VA VDD 2/3 × VDD 1/3 × VDD VDD on VB 3/2 × VDD VDD 1/2 × VDD

VREFIN on VC 3 × VREFIN 2 × VREFIN VREFIN

C Type Bias Power Supply Scheme

The connection to the VMAX pin depends upon the LCD power supply scheme. It is

extremely important to ensure that these charge pump generated internal voltages do not

exceed the maximum VDD voltage of 5.5V.

Condition VMAX Pin Connection

VDD > VPLCD × 1.5 Connect VMAX to VDD

Otherwise Connect VMAX to V1

C Type Bias VMAX Pin Connection


AN0410E V1.00 10 / 13 December 11, 2016

LCD Driver with SCOM Function Take the HT66Fxx series as an example, the LCD common pins SCOM0~SCOM3 are

pin-shared with PC0~PC3 or PC0~PC1, PC6~PC7. The LCD control signals are

implemented by the application program.

LCD Operation

An external LCD panel can be driven using the device by configuring the PC0~PC3 or

PC0~PC1, PC6~PC7 as common pins and using other output port lines as segment pins.

The LCD driver function is controlled using the SCOMC register which in addition to

controlling the overall on/off function also controls the bias voltage setup function. This

enables the LCD COM driver to generates the necessary VDD/2 voltage level for LCD 1/2

bias operation.

The SCOMEN bit in the SCOMC register is the overall master control for the LCD driver,

however this bit is used in conjunction with the COMnEN bits to select which Port C pins

are used for LCD driving. Note that the Port Control register does not need to first setup

the pins as outputs to enable the LCD driver operation.

SCOM operating current

SCOM0~SCOM3

SCOMEN

VDD/2

VDD

COMnEN

LCD COM Bias

SCOMEN COMnEN Pin Function O/P Level

0 X I/O 0 or 1

1 0 I/O 0 or 1

1 1 SCOMn VDD/2

Output Control


AN0410E V1.00 11 / 13 December 11, 2016

LCD Bias Control

The LCD driver enables a range of selections to be provided to suit the requirement of the

LCD panel which is being used. The bias resistor choice is implemented using the ISEL1

and ISEL0 bits in the SCOMC register.

Bit 7 6 5 4 3 2 1 0

Name D4 ISEL1 ISEL0 SCOMEN COM3EN COM2EN COM1EN COM0EN

R/W R/W R/W R/W R/W R/W R/W R/W R/W

POR 0 0 0 0 0 0 0 0

LCD Driver with SCOM and SSEG Functions Take the HT66F0175/HT66F0185 series as an example，the devices have the capability

of driving external LCD panels. The common and segment pins for LCD driving,

SCOM0~SCOM5 and SSEG0~SEEG19 or SSEG0~SEEG23, are pin-shared with certain

pins on the I/O ports. The LCD signals, COM and SEG, are generated using the

application program.

LCD Operation

An external LCD panel can be driven using the device by configuring the I/O pins as

common pins and segment pins. The LCD driver function is controlled using the LCD

control registers which in addition to controlling the overall on/off function also controls

the R type bias current on the SCOM and SSEG pins. This enables the LCD COM and

SEG driver to generate the necessary VSS, (1/3)VDD, (2/3)VDD and VDD voltage levels for

LCD 1/3 bias operation.

The LCDEN bit in the SLCDC0 register is the overall master control for the LCD driver.

This bit is used in conjunction with the COMnEN and SEGmEN bits to select which I/O

pins are used for LCD driving. Note that the corresponding Port Control register does not

need to first setup the pins as outputs to enable the LCD driver operation.

VDD

(2/3) VDD

(1/3) VDD

VDD

LCDVoltageSelectCircuit

LCDCOM/SEG

Analog Switch

LCDEN

ISEL[1:0]

COMnEN COMSEGSn66

SEGmEN

FRAME

SCOM0/SSEG0

SCOM5/SSEG5

SSEG6

SSEGm

m = 19 for HT66F0175m = 23 for HT66F0185

Software Controlled LCD Driver Structure


AN0410E V1.00 12 / 13 December 11, 2016

LCD Control Registers

The LCD COM and SEG driver enables a range of selections to be provided to suit the

requirement of the LCD panel which is being used. The bias resistor choice is

implemented using the ISEL1 and ISEL0 bits in the SLCDC0 register. All COM and SEG

pins are pin-shared with I/O pins and are selected using the corresponding pin function

selection bit in the SLCDCn register.

Register Name

Bit

7 6 5 4 3 2 1 0

SLCDC0 FRAME ISEL1 ISEL0 LCDEN COM3EN COM2EN COM1EN COM0EN

SLCDC1 COM5EN COM4EN COMSEGS5 COMSEGS4 COMSEGS3 COMSEGS2 COMSEGS1 COMSEGS0

SLCDC2 SEG13EN SEG12EN SEG11EN SEG10EN SEG9EN SEG8EN SEG7EN SEG6EN

SLCDC3 — — SEG19EN SEG18EN SEG17EN SEG16EN SEG15EN SEG14EN

LCD Driver Control Registers List − HT66F0175

Register Name

Bit

7 6 5 4 3 2 1 0

SLCDC0 FRAME ISEL1 ISEL0 LCDEN COM3EN COM2EN COM1EN COM0EN

SLCDC1 COM5EN COM4EN COMSEGS5 COMSEGS4 COMSEGS3 COMSEGS2 COMSEGS1 COMSEGS0



SLCDC4 — — — — — — SEG23EN SEG22EN

LCD Driver Control Registers List − HT66F0185

Conclusions This guideline has introduced several Holtek MCU LCD driving schemes. The C type

scheme uses an internal charge pump to provide the bias voltage with lower power

consumption. The R type scheme uses the bias registers to provide the bias voltage with

higher power consumption. The remaining two types are the SCOM scheme and the

SCOM and SSEG scheme.


AN0410E V1.00 13 / 13 December 11, 2016

Versions and Modify Information

Date Author Issue Release and Modification

2015.12.15 David, Xue (薛明列) First Version

Reference Files

Reference files: HT66F40, HT66F0175/0185 and BS67F350 DataSheet.

For more information, refer to the Holtek’s official website www.holtek.com.

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HT8 MCU Integrated LCD Application Guideline - holtek.com · HT8 MCU Integrated LCD Application...

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