Silicon Laboratories EFM32 User manual

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.
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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
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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.
AC
AC
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
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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
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
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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
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VLCD
1/2VLCD
VSS
VLCD
1/2VLCD
VSS
VLCD
1/2VLCD
VSS
VSS
VLCD
1/2VLCD
VLCD
0 V
-VLCD
1/2VLCD
-1/2VLCD
VLCD
0 V
-VLCD
1/2VLCD
-1/2VLCD
On
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.
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Introduction to LCD Segment Displays
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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
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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
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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
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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
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
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