JBL Bar 9.1 Technical Data
AN0057.0: EFM32 Series 0 LCD Driver
This application note provides a description of how passive segment 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
• Multiple IDE projects
KEY FEATURES
• The ultra-low power LCD driver has
internal bias voltage circuit and boost
converter to minimize external components.
• The voltage boost function enables it to
provide the LCD display with higher
voltage than the supply voltage of the
device.
• The LCD driver supports autonomous
animation and blinking in deep sleep
without CPU intervention.
silabs.com | Building a more connected world. Rev. 1.05
AN0057.0: EFM32 Series 0 LCD Driver
1. Device Compatibility
This application note supports multiple device families, and some functionality is different depending on the device.
EFM32 MCU Series 0 consists of:
• EFM32 Gecko (EFM32G)
• EFM32 Tiny Gecko (EFM32TG)
• EFM32 Giant Gecko (EFM32GG)
• EFM32 Leopard Gecko (EFM32LG)
• EFM32 Wonder Gecko (EFM32WG)
Device Compatibility
silabs.com | Building a more connected world. Rev. 1.05 | 2
AN0057.0: EFM32 Series 0 LCD Driver
Introduction to LCD Segment Displays
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 between 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 voltage.
AC
AC
Liquid
Crystal
Liquid
Crystal
Non-
polarized
light
Segment Off
Polarizer
Electrodes
90° Polarizer
Reflective Layer
Non-
polarized
light
Segment On
Polarizer
Electrodes
90° Polarizer
Reflective Layer
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.
silabs.com | Building a more connected world. Rev. 1.05 | 3
AN0057.0: EFM32 Series 0 LCD Driver
Introduction to LCD Segment Displays
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
V
LCD
V
SS
V
LCD
V
SS
V
LCD
V
SS
V
LCD
0 V
-V
LCD
V
LCD
0 V
-V
LCD
Segment 0 Segment 1
COM0
SEG0
SEG1
V
RMS
Resulting Voltage COM0-SEG0
V
RMS
Resulting Voltage COM0-SEG1
On Off
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.
silabs.com | Building a more connected world. Rev. 1.05 | 4
AN0057.0: EFM32 Series 0 LCD Driver
Introduction to LCD Segment Displays
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"-electrode. See the following figure.
Segment
electrode 0
Common
backplane
Liquid
Crystal
OnOff
Segment
electrode 1
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 number 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.
silabs.com | Building a more connected world. Rev. 1.05 | 5
Loading...