MSP430FR6889 Datasheet, PDF (155/176 Pages) Texas Instruments – Mixed-Signal Microcontrollers

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MSP430FR6889, MSP430FR68891, MSP430FR6888, MSP430FR6887
MSP430FR5889, MSP430FR58891, MSP430FR5888, MSP430FR5887
SLASE32A – AUGUST 2014 – REVISED MARCH 2015
7.2.2.3 Detailed Design Procedure
A major component in designing the LCD solution is determining the exact connections between the
LCD_C 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 recommended:
• 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_C 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_C 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 of the most convenient memory layout needs to 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.
For design information regarding the LCD controller input voltage selection including internal and external
options, contrast control, and bias generation, refer to the LCD_C controller chapter in the
MSP430FR58xx, MSP430FR59xx, MSP430FR68xx, and MSP430FR69xx Family User's Guide
(SLAU367).
For additional design information, see the application report Designing With MSP430 and Segment LCD
(SLAA654).
7.2.2.4 Layout Guidelines
LCD segment (Sx) and common (COMx) signal traces are continuously switching while the LCD is
enabled and should, therefore, be kept away from sensitive analog signals such as ADC inputs to prevent
any noise coupling. TI recommends keeping the LCD signal traces on one side of the PCB grouped
together in a bus-like fashion. A ground plane underneath the LCD traces and guard traces employed
alongside the LCD traces can provide shielding.
If the internal charge pump of the LCD module is used, the externally provided capacitor on the LCDCAP
pin should be located as close as possible to the MCU. The capacitor should be connected to the device
using a short and direct trace and also have a solid connection to the ground plane that is supplying the
V_SS pins of the MCU.
For an example layout of connecting a 4-mux LCD with 40 segments to an MSP430FR6989 and using the
charge pump feature, see the Gas or Water Meter With Two LC Sensors reference design (TIDM-LC-
WATERMTR).
Copyright © 2014–2015, Texas Instruments Incorporated
Applications, Implementation, and Layout 155
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