MSP430FG6626 Datasheet, PDF (158/171 Pages) Texas Instruments – Mixed-Signal Microcontrollers

MSP430FG6626, MSP430FG6625
MSP430FG6426, MSP430FG6425
SLAS874 – MAY 2015
www.ti.com
OPTION OR FEATURE
Multiplexed LCD
Static LCD
Internal Bias Generation
External Bias Generation
Internal Charge Pump
IMPACT OR USE CASE
• Enable displays with more segments
• Use fewer device pins
• LCD contrast decreases as mux level increases
• Power consumption increases with mux level
• Requires multiple intermediate bias voltages
• Limited number of segments that can be addressed
• Use a relatively large number of device pins
• Use the least amount of power
• Use only V_CC and GND to drive LCD signals
• Simpler solution – no external circuitry
• Independent of V_LCD source
• Somewhat higher power consumption
• Requires external resistor ladder divider
• Resistor size depends on display
• Ability to adjust drive strength to optimize tradeoff between power consumption and good drive of large
segments (high capacitive load)
• External resistor ladder divider can be stabilized through capacitors to reduce ripple
• Helps ensure a constant level of contrast despite decaying supply voltage conditions (battery-powered
applications)
• Programmable voltage levels allow software-driven contrast control
• Requires an external capacitor on the LCDCAP pin
• Higher current consumption than simply using V_CC for the LCD driver
7.2.4.3 Detailed Design Procedure
A major component in designing the LCD solution is determining the exact connections between the
LCD_B peripheral module and the display itself. Two basic design processes can be employed for this
step, although in reality often a balanced co-design approach is necessary:
• PCB layout-driven design
• Software-driven design
In the PCB layout-driven design process, the segment Sx and common COMx signals are connected to
respective MSP430 device pins so that the routing of the PCB can be optimized to minimize signal
crossings and to keep signals on one side of the PCB only, typically the top layer. For example, using a
multiplexed LCD, it is possible to arbitrarily connect the Sx and COMx signals between the LCD and the
MSP430 device as long as segment lines are swapped with segment lines and common lines are
swapped with common lines. It is also possible to not contiguously connect all segment lines but rather
skip LCD_B module segment connections to optimize layout or to allow access to other functions that may
be multiplexed on a particular device port pin. Employing a purely layout-driven design approach,
however, can result in the LCD_B module control bits that are responsible for turning on and off segments
to appear scattered throughout the memory map of the LCD controller (LCDMx registers). This approach
potentially places a rather large burden on the software design that may also result in increased energy
consumption due to the computational overhead required to work with the LCD.
The other extreme is a purely software-driven approach that starts with the idea that control bits for LCD
segments that are frequently turned on and off together should be co-located in memory in the same
LCDMx register or in adjacent registers. For example, in case of a 4-mux display that contains several 7-
segment digits, from a software perspective it can be very desirable to control all 7 segments of each digit
though a single byte-wide access to an LCDMx register. And consecutive segments are mapped to
consecutive LCDMx registers. This allows use of simple look-up tables or software loops to output
numbers on an LCD, reducing computational overhead and optimizing the energy consumption of an
application. Establishing the most convenient memory layout must be performed in conjunction with the
specific LCD that is being used to understand its design constraints in terms of which segment and which
common signals are connected to, for example, a digit.
158 Applications, Implementation, and Layout
Copyright © 2015, Texas Instruments Incorporated
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