LCD_C Operation
940
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
LCD_C Controller
36.2.5.3 LCD Contrast Control
The peak voltage of the output waveforms together with the selected mode and biasing determine the
contrast and the contrast ratio of the LCD. The LCD contrast can be controlled in software by adjusting the
LCD voltage generated by the integrated charge pump using the VLCDx settings.
The contrast ratio depends on the used LCD display and the selected biasing scheme.
shows
the biasing configurations that apply to the different modes together with the RMS voltages for the
segments turned on (V
RMS,ON
) and turned off (V
RMS,OFF
) as functions of V
LCD
. It also shows the resulting
contrast ratios between the on and off states.
(1)
Recommended setting to achieve best contrast
Table 36-3. LCD Voltage and Biasing Characteristics
Mode
Bias
Config
LCDMx
LCD2B
COM
Lines
Voltage Levels
V
RMS,OFF
/ V
LCD
V
RMS,ON
/ V
LCD
Contrast
Ratio V
RMS,ON
/
V
RMS,OFF
Static
Static
0000
X
1
V1, V5
0
1
1/0
2-mux
1/2
(1)
0001
1
2
V1, V3, V5
0.354
0.791
2.236
1/3
0001
0
2
V1, V2, V4, V5
0.333
0.745
2.236
3-mux
1/2
0010
1
3
V1, V3, V5
0.408
0.707
1.732
1/3
(1)
0010
0
3
V1, V2, V4, V5
0.333
0.638
1.915
4-mux
1/2
0011
1
4
V1, V3, V5
0.433
0.661
1.528
1/3
(1)
0011
0
4
V1, V2, V4, V5
0.333
0.577
1.732
5-mux
1/3
(1)
0100
0
5
V1, V2, V4, V5
0.333
0.537
1.612
6-mux
1/3
(1)
0101
0
6
V1, V2, V4, V5
0.333
0.509
1.528
7-mux
1/3
(1)
0110
0
7
V1, V2, V4, V5
0.333
0.488
1.464
8-mux
1/3
(1)
0111
0
8
V1, V2, V4, V5
0.333
0.471
1.414
A typical approach to determine the required V
LCD
is by equating V
RMS,OFF
with a LCD threshold voltage
provided by the LCD manufacturer, for example when the LCD exhibits approximately 10% contrast
(V
th,10%
): V
RMS,OFF
= V
th,10%
. Using the values for V
RMS,OFF
/V
LCD
provided in the table results in V
LCD
=
V
th,10%
/(V
RMS,OFF
/V
LCD
). In the static mode, a suitable choice is V
LCD
greater than or equal to three times
V
th,10%
.
In 3-mux and 4-mux mode, a 1/3 biasing is typically used, but a 1/2 biasing scheme is also possible. The
1/2 bias reduces the contrast ratio, but the advantage is a reduction of the required full-scale LCD voltage
V
LCD
.
36.2.6 LCD Outputs
Some LCD segment, common, and Rxx functions are multiplexed with digital I/O functions. These pins
can function either as digital I/O or as LCD functions.
The LCD segment functions, when multiplexed with digital I/O, are selected using the LCDSx bits in the
LCDCPCTLx registers. The LCDSx bits select the LCD function for each segment line. When LCDSx = 0,
a multiplexed pin is set to digital I/O function. When LCDSx = 1, a multiplexed pin is selected as LCD
function.
The pin functions for COMx and Rxx, when multiplexed with digital I/O, are selected as described in the
port schematic section of the device-specific data sheet. An some devices the COM1 to COM7 pins are
shared with segment lines, refer to the device-specific data sheet. If these pins are required as COM pins
due to the selected LCD multiplexing mode, the COM functionality takes precedence over the segment
function that can be selected for those pins with the LCDSx bits as for all other segment pins.
See the port schematic section of the device-specific data sheet for details on controlling the pin
functionality.