Silicon Laboratories EFM32 Bedienungsanleitung

AN0057.1: EFM32 Series 1 LCD Driver

This application note provides a description of how passive seg-

ment LCD displays work and how they can be interfaced with the

EFM32.

This application note includes:

• This PDF document

• Source files (zip)

• Example C-code

• Mulitple IDE projects

KEY FEATURES

• The ultra-low power LCD driver has

internal bias voltage circuit to minimize

external components.

• The charge pump mode enables it to

provide the LCD display up to twice the

supply voltage of the device.

• The power consumption of the LCD panel

itself can be lowered through the use of

charge redistribution.

• The LCD driver supports autonomous

animation and blinking in deep sleep

without CPU intervention.

silabs.com | Building a more connected world. Rev. 1.05

1. Device Compatibility

This application note supports multiple device families, and some functionality is different depending on the device.

EFM32 MCU Series 1 consists of:

• EFM32 Giant Gecko (EFM32GG11)

• EFM32 Tiny Gecko (EFM32TG11)

AN0057.1: EFM32 Series 1 LCD Driver

Device Compatibility

silabs.com | Building a more connected world. Rev. 1.05 | 2

2. Introduction to LCD Segment Displays

Segmented liquid crystal displays (LCDs) are a common way to display information. The extreme low-power LCD driver in the EFM32

enables many applications to use an LCD even in energy critical systems. This document will both discuss how certain types of LCDs

work, how they are driven and how to minimize their energy consumption.

2.1 Passive LCD Displays

This document will only discuss passive segment LCDs. These displays are usually constructed by sandwiching the liquid crystal be-

tween two glass plates. By using the voltage dependent, polarizing properties of the liquid crystal material, light transmission through

the LCD glass can be controlled. The display is usually built up by segments that can either block light or let it pass through depending

on the voltage applied over the liquid crystal within that segment.

By having a reflective coating on one side of the glass, ambient light can either be reflected back to the user or blocked by the polarizer

closest to the reflective layer. This blocking of light occurs because the liquid crystal in one state will change the polarization of the light

to allow it to pass both polarizers. In the other state it will not affect the polarization, and the light is then blocked because the two

polarizers are orthogonally oriented with respect to each other.

This document will further use the notion that a segment is "on" when it is blocking light and "off" when light can pass through. Some

displays invert this notion by letting light pass through the "on" segments, and having the "off" segments block light. The figure below

illustrates how light polarization is affected when light passes through the polarizers and liquid crystal with and without an applied volt-

age.

Electrodes

Polarizer

90° Polarizer

Reflective Layer

Liquid

Crystal

Electrodes

Polarizer

90° Polarizer

Reflective Layer

Liquid

Crystal

Non-

polarized

light

Non-

polarized

light

Segment On

Segment Off

Figure 2.1. One LCD Segment with and Without Voltage Applied

When the voltage is applied, the liquid crystal does not change the polarization direction of the light. This causes it to be blocked by the

second polarizer, which is oriented at an orthogonal angle to the first polarizer. If no voltage is applied, the liquid crystal will change the

polarization of the light so that it can pass through both polarizers.

AN0057.1: EFM32 Series 1 LCD Driver

Introduction to LCD Segment Displays

silabs.com | Building a more connected world. Rev. 1.05 | 3

2.2 Driving a Display Segment

Segment LCDs do not have any internal driving circuitry. All the display pins are directly connected to each side of the corresponding

segment. The simplest LCD imaginable consist of only one segment with two electrical connections, one for each side of the segment.

The segment is turned on or off by applying a voltage across it. The electric field, or voltage, between the top and bottom of the liquid

crystal between the glass plates directly affects the polarization properties, see Figure 2.1 One LCD Segment with and Without Voltage

Applied on page 3.

Just applying a constant voltage across the segment to turn it on would work, but not for a long time. The problem is that after a short

time the LCD segment will be affected by the DC-current passing through it, mainly through electrolysis effects on the liquid crystal and

electrodes. The solution is to drive the segment with a waveform that has an average 0 DC value. As long as the voltage is switched

fast enough (30 - 100 Hz), it will be the RMS value of the voltage that affects the amount of polarization.

The simplest way to drive a "one segment" LCD with the correct zero DC bias would be to apply two identical and optionally phase

shifted, square waveforms. By shifting the phase of one signal with respect to the other by 180 degrees, the apparent RMS voltage

across the segment goes from 0 to two times the voltage of the two waveforms. See the following figure.

Frame Start Frame End

VLCD

VSS

VLCD

VSS

VLCD

VSS

0 V

-VLCD

VLCD

0 V

-VLCD

VLCD

On Off

Resulting Voltage COM0-SEG0

Resulting Voltage COM0-SEG1

Segment 0 Segment 1

COM0

SEG0

SEG1

VRMS

Figure 2.2. Static Driving of Two LCD Segments, One On and One Off

The reason for using two square waveforms, offset from 0 V is that this is simpler to achieve on a microcontroller with a single power

supply. Negative voltages are rarely available, so connecting one side of the LCD segments to ground is not an option.

AN0057.1: EFM32 Series 1 LCD Driver

Introduction to LCD Segment Displays

silabs.com | Building a more connected world. Rev. 1.05 | 4

2.2.1 Driving Many Segments: Static Driving

An LCD display most often has more than one segment. Usually the segments are connected with one side common to many of the

other segments, this is called the "common"-electrode. The other side of the segment has its own pin connection, the "segment"-elec-

trode. See the following figure.

Segment

electrode 1

Segment

electrode 0

Common

backplane

OnOff Liquid

Crystal

Figure 2.3. Common Backplane with Two Segments, One On and One Off

With a statically driven LCD display, there is only one common electrode and each segment has its own segment electrode. Both the

common electrode and segments are driven with a square wave form. Segments that are "on" are driven with a phase shifted waveform

to make the RMS voltage across these segment non-zero. See Figure 2.2 Static Driving of Two LCD Segments, One On and One Off

on page 4.

2.2.2 Driving Many Segments: Multiplexed Driving

With the static driving approach with one segment line for each segment, large displays with many segments would need a large num-

ber of the microcontroller pins just to drive the display. By multiplexing several common and segment lines, fewer pins can be used to

drive more segments. The total number of segments that can be driven is the product of the number of common lines and segment

lines. Usually maximum contrast goes down and current consumption goes up with a higher number of common lines.

It is the amplitude of the apparent RMS voltage across a segment that determines if it is on or off. A segment with a low RMS voltage

applied will seem to be off even if the voltage is non-zero. The relationship between apparent RMS voltage across a segment and its

visual properties is non-linear. This non-linearity is utilized when multiplexing several segments on the same driving pins. A segment

does not need to see zero RMS voltage to be perceived as completely off by the user.

Each common line and segment line is driven with waveforms consisting of more than two voltage levels as in the static driving case.

The number of voltage levels are known as "bias" levels. By carefully selecting the waveform of each segment line and common line, it

is possible to make some segments "see" a low RMS voltage, while others "see" a high RMS voltage, even if the segments share either

their common or segment electrode with other segments. See the figure below.

AN0057.1: EFM32 Series 1 LCD Driver

Introduction to LCD Segment Displays

silabs.com | Building a more connected world. Rev. 1.05 | 5

VLCD

1/2VLCD

VSS

VLCD

1/2VLCD

VSS

VLCD

1/2VLCD

VSS

VLCD

1/2VLCD

VLCD

0 V

-VLCD

1/2VLCD

-1/2VLCD

VLCD

0 V

-VLCD

1/2VLCD

-1/2VLCD

Off On

Off

COM0

COM1

SEG0

SEG1

Resulting Voltage SEG0-COM0

Resulting Voltage SEG0-COM1

Frame Start Frame End

Segment 0 Segment 1

Segment 2 Segment 3

Figure 2.4. Multiplexed Driving of Four LCD Segments

In the figure above there are two segment lines and two common lines. The bias is 1/2, which means there are 3 different voltage levels

possible. The combination of the waveform on COM0 and SEG0 makes the segment between SEG0 and COM0 turn on, while the seg-

ment between SEG0 and COM1 is off. Notice the difference in RMS voltage across the two segments.

For a multiplexed display, the difference in RMS voltage between segments that are on and off will be smaller than for a statically driven

display. This means that the visual quality and appearance, such as contrast and viewing angle, must be more carefully calibrated for a

multiplexed display. The contrast can be calibrated in software by adjusting the bias voltage levels within the LCD driver. Viewing angle

and appearance under different lighting conditions should be considered when selecting the actual display.

AN0057.1: EFM32 Series 1 LCD Driver

Introduction to LCD Segment Displays

silabs.com | Building a more connected world. Rev. 1.05 | 6

3. EFM32 LCD Driver

This chapter explains some of the features and possibilities of the EFM32 built-in LCD driving peripheral. For a full set of features and

limitations, please see the reference manual for the specific device family.

3.1 Multiplexing

The EFM32 LCD driver can multiplex up to 8 common lines, of which 4 are dedicated common lines and 4 are part of the segment lines

(not available on all parts). The multiplexing waveforms and bias voltage levels are automatically handled within the LCD driver periph-

eral. The software only needs to set segments on or off and adjust contrast if necessary.

If multiplexing is not needed, static driving of the display is also possible. A statically-driven display can have higher contrast than a

multiplexed display. A static display with only one common line is usually lower power than a multiplexed display, and, in most cases,

current consumption goes up with a larger number of common lines.

3.2 Waveform and Bias Selection

When selecting multiplex mode and number of bias levels according to how many common lines the LCD has, the waveforms are se-

lected automatically by the EFM32 LCD peripheral. Normal and low power waveform modes are selectable with the difference being

that the low power mode achieves zero DC bias over several frames. In other words, the DC bias across LCD segments is not zero

within a frame. This is not a problem for most displays and applications and is the mode almost always recommended.

Figure 3.1 Normal and Low Power Waveform, Only Common Signal Shown on page 7 shows the difference between low power and

normal mode and its larger number of waveform changes within a frame. If flickering or contrast problems occur are observed in low-

power mode, normal mode can be selected instead. An illustration of the different waveforms used, up to and including quadplexed

mode, can be found in the reference manual.

VLC0 (VLCD)

VLC1 (2/3VLCD)

VLC3 (VSS)

VLC2 (1/3VLCD)

Frame Start Frame End

VLC0 (VLCD)

VLC1 (2/3VLCD)

VLC3 (VSS)

VLC2 (1/3VLCD)

Frame Start Frame End

Normal Waveform

Low Power Waveform

Figure 3.1. Normal and Low Power Waveform, Only Common Signal Shown

AN0057.1: EFM32 Series 1 LCD Driver

EFM32 LCD Driver

silabs.com | Building a more connected world. Rev. 1.05 | 7

Selecting the correct bias is necessary to provide the waveform generator with enough voltage levels to produce the necessary wave-

form for the chosen number of common lines. In general 5- to 8-plexed modes need 1/4 bias (5 voltage levels), 3- to 4-plexed modes

need 1/3 bias (4 voltage levels), while 2-plexed mode needs 1/2 bias (3 voltage levels) and static mode needs only 2 voltage levels.

The following table illustrates the need for more bias levels when increasing the number of multiplexed common lines. The difference

between Von and Voff is essentially a measurement of the possible contrast. Notice that for the static driven display, the contrast is es-

sentially infinite. Refer to Figure 3.2 Relative Transmission of Light Through the LCD for Changes in the Applied RMS Voltage on page

9 for an illustration of the non-linear properties of the liquid crystal and why the difference between Von and Voff is a measure of

contrast.

Table 3.1. LCD Drive Modes, Bias Setting and Contrast Characteristics

LCD drive mode configuration LCD bias setting Contrast = Von(RMS)/Voff(RMS)

Static Static Infinite

1:2 multiplex 1/2 2.24

1:2 multiplex 1/3 2.24

1:2 multiplex 1/4 1.85

1:4 multiplex 1/2 1.53

1:4 multiplex 1/3 1.73

1:4 multiplex 1/4 1.65

1:6 multiplex 1/2 1.34

1:6 multiplex 1/3 1.53

1:6 multiplex 1/4 1.53

1:8 multiplex 1/2 1.25

1:8 multiplex 1/3 1.41

1:8 multiplex 1/4 1.45

Note that for some multiplex configurations, contrast actually goes down with an increased number of bias levels. In general a higher

number of bias levels also means higher energy consumption, so for a given number of multiplexed common lines, the lowest possible

number of bias levels should be chosen. Depending on the LCD, contrast might be acceptable even with a lower number of bias levels.

AN0057.1: EFM32 Series 1 LCD Driver

EFM32 LCD Driver

silabs.com | Building a more connected world. Rev. 1.05 | 8

3.3 Contrast

Adjusting the contrast of the LCD is only relevant to multiplexed displays because the off segments in multiplexed mode still have a

non-zero RMS voltage across them. The purpose of contrast adjustment is to get the RMS voltage of off segments below a certain

threshold so these segments are perceived as off but there is still a high enough voltage across the on segments for them to be per-

ceived as on. Contrast varies among LCDs and also changes with temperature and voltage, thus making adjustments necessary. See

Figure 3.2 Relative Transmission of Light Through the LCD for Changes in the Applied RMS Voltage on page 9 for an illustration of

the non-linear nature of the liquid crystal material with respect to applied voltage and transmission of light without changing polarization.

Relative

Transmission

10%

90%

100%

VRMS [V]

Segment off Segment grey Segment on

Figure 3.2. Relative Transmission of Light Through the LCD for Changes in the Applied RMS Voltage

3.3.1 Charge Pump Mode

Some LCDs require higher voltage waveforms to work. The EFM32 Series 1 LCD peripheral supports charge pump mode to generate a

VLCD voltage up to twice the supply voltage of the device. VLCD is adjusted by the LCD_DISPCTRL register CONTRAST[5:0] field.

The usual sign that charge pump mode is needed is that LCD contrast goes down and on segments appear dim even with the contrast

adjusted to its highest level in other modes. Reduced contrast and even perceived blanking of the display is also a problem at low tem-

peratures and can be resolved in software by enabling charge pump mode to drive VLCD at a higher voltage level.

See the reference manual for more information on the external components necessary for using the charge pump and how to enable

and adjust its output voltage.

3.3.2 Contrast Compensation

As mentioned earlier, LCD contrast is affected by both the EFM32 supply voltage and the temperature of the LCD glass. If the system is

intended for operation over a wide range of supply voltages and temperatures, the contrast, and optionally the use of charge pump

mode, should be adjusted under software control at predetermined thresholds.

The correct contrast adjustment varies between different displays and viewing angles. Exactly at which voltages and temperatures the

charge pump mode is needed is also dependent on the LCD display itself. Some information can often be found in the LCD glass docu-

mentation.

AN0057.1: EFM32 Series 1 LCD Driver

EFM32 LCD Driver

silabs.com | Building a more connected world. Rev. 1.05 | 9

3.4 Animation and Blinking

The EFM32 LCD driver includes special features to enable animation and blinking of specific segments without any software interven-

tion. This is useful for displaying continuous animation to signal that a device is alive, without the need to wake up from deep sleep to

update the segments.

The animation feature is available on segments 0 to 7 (or segments 8 to 15) multiplexed with LCD_COM0. The animation is implemen-

ted as two programmable 8-bit registers that are shifted left or right every other Animation state for a total of 16 states. The animation

state changes with each frame counter event. See the reference manual for more information on the animation feature and its interface.

The animation registers and shift operation are shown in the following image.

Seg7

AREGB7

AREGA7

ALOGSEL

Seg6

AREGB6

AREGA6

Seg0

AREGB0

AREGA0

Barrel shift right/left

Figure 3.3. Animation Function with Barrel Shift and Logic Operations

The included software example demonstrates how to enable and configure the blinking and animation features.

3.5 LCD Interrupts

The LCD peripheral contains one interrupt source, the frame counter interrupt. It can be used to time and execute display updates,

animation changes or similar display effects. Since there are no limitations on when the LCD segment registers can be updated, there

is no need to use the frame counter interrupt to time segment updates. The driver supplied in the Gecko SDK Suite and which is used

in the software example with this application note does not use this interrupt. See the reference manual for more information.

3.6 Minimize Energy Consumption

The LCD driver is itself a very low power peripheral that can operate in deep sleep mode (EM2) from a low frequency oscillator. When

connecting a display to the EFM32, the capacitive load presented by the LCD results in increased energy consumption. This can be

minimized by optimizing a few parameters.

After the multiplex and bias settings are configured correctly, and low power waveform is selected, the display refresh rate should be

configured to the lowest possible value that does not cause flickering by setting the frame rate division and prescale values.

Charge pump mode should not be used unless absolutely necessary. Instead, contrast should first be adjusted to counteract lower sup-

ply voltage or temperatures. Remember that charge pump mode significantly increases current consumption.

In addition, the power consumption of the LCD panel itself can be lowered through the use of charge redistribution. When charge redis-

tribution is in use, all segments are briefly shorted together, allowing low segments to be partially charged by the energy in high seg-

ments instead of using additional energy from the power supply. With charge redistribution, the EFM32 LCD peripheral can reduce cur-

rent consumption by 10% to 24% in the low power waveform mode and by up to 40% in the normal waveform mode. The included

software example demonstrates charge redistribution and shows the reduction in current draw when it is enabled.

CHGRDST[1:0] in LCD_DISPCTRL sets the charge redistribution duty cycle, which should be 5% or less to prevent any reduction in

LCD pin RMS voltage. See the reference manual for for how to calculate and set charge redistribution duty cycle.

If charge redistribution is not used, a larger CMU prescaling value is recommended to minimize power consumed by the LCD block

itself. Note that disabling charge redistribution will always result in higher system power consumption. Charge redistribution is on by

default, but it can be disabled by setting CHGRDST to 0 in LCD_DISPCTRL register.

AN0057.1: EFM32 Series 1 LCD Driver

EFM32 LCD Driver

silabs.com | Building a more connected world. Rev. 1.05 | 10

Andere Handbücher für EFM32

Inhaltsverzeichnis

Andere Silicon Laboratories Mikrocontroller Handbücher

Silicon Laboratories

Silicon Laboratories AN1342 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F930-DK Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051xxxDK Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories CP2102N-MINIEK Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories SiM3U1xx Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F00 DK Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories Wireless M-Bus-EK Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F326-DK Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories EFM32TG Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F12 Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories EFM8UB1-SLSTK2000A Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories Si8410 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F310-TB Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories TOOLSTICK C8051F850 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories BG22 Explorer Kit Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories Si8410 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F300 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories Si4010 Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories EZRadioPRO Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories UG464 Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F34-DK Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F310DK-T Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories EFM32 EFM32GG11 Giant Gecko Series Bedienungsanleitung

Silicon Laboratories

Silicon Laboratories C8051F32x-DK Bedienungsanleitung

Beliebte Mikrocontroller Handbücher anderer Marken

AMS

AMS AS7261 Demo Kit Bedienungsanleitung

Novatek

Novatek NT6861 Bedienungsanleitung

Espressif Systems

Espressif Systems ESP8266 SDK Einrichtungshandbuch

Nuvoton

Nuvoton ISD61S00 ChipCorder Leitfaden

STMicrolectronics

STMicrolectronics ST7 Assembler Linker Bedienungsanleitung

Texas Instruments

Texas Instruments Chipcon CC2420DK Bedienungsanleitung

Lantronix

Lantronix Intrinsyc Open-Q 865XR SOM Bedienungsanleitung

NEC

NEC 78GK0S/K 1+ Series Montage- und Bedienungsanleitung

Mikroe

Mikroe SEMITECH N-PLC Click Montage- und Bedienungsanleitung

Intel

Intel Agilex Bedienungsanleitung

Cypress

Cypress CY4607 HX2VL Bedienungsanleitung

DIGITAL-LOGIC

DIGITAL-LOGIC MICROSPACE Bedienungsanleitung

Texas Instruments

Texas Instruments TMS320F2837 D Series Bedienungsanleitung

CYPRES

CYPRES CY14NVSRAMKIT-001 Bedienungsanleitung

Texas Instruments

Texas Instruments INA-DUAL-2AMP-EVM Bedienungsanleitung

Espressif Systems

Espressif Systems ESP8266EX Betriebsanleitung

Abov

Abov AC33M8128L Bedienungsanleitung

Laird

Laird BL654PA Bedienungsanleitung