6.2.1.1 Partial Schematic

Figure 6-5 shows the recommended decoupling circuit when an external voltage reference is used.

Figure 6-5 ADC12_B Grounding and Noise Considerations

6.2.1.2 Design Requirements

As with any high-resolution ADC, appropriate printed-circuit-board layout and grounding techniques should be followed to eliminate ground loops, unwanted parasitic effects, and noise.

Ground loops are formed when return current from the ADC flows through paths that are common with other analog or digital circuitry. If care is not taken, this current can generate small unwanted offset voltages that can add to or subtract from the reference or input voltages of the ADC. The general guidelines in Section 6.1.1 combined with the connections shown in Section 6.2.1.1 prevent this.

In addition to grounding, ripple and noise spikes on the power-supply lines that are caused by digital switching or switching power supplies can corrupt the conversion result. A noise-free design using separate analog and digital ground planes with a single-point connection is recommend to achieve high accuracy.

Figure 6-5 shows the recommended decoupling circuit when an external voltage reference is used. The internal reference module has a maximum drive current as specified in the Reference module's I_O(VREF+) specification.

The reference voltage must be a stable voltage for accurate measurements. The capacitor values that are selected in the general guidelines filter out the high- and low-frequency ripple before the reference voltage enters the device. In this case, the 10-µF capacitor is used to buffer the reference pin and filter any low-frequency ripple. A bypass capacitor of 4.7 µF is used to filter out any high-frequency noise.

6.2.1.3 Detailed Design Procedure

For additional design information, see Designing With the MSP430FR58xx, FR59xx, FR68xx, and FR69xx ADC.

6.2.1.4 Layout Guidelines

Component that are shown in the partial schematic (see Figure 6-5) should be placed as close as possible to the respective device pins. Avoid long traces, because they add additional parasitic capacitance, inductance, and resistance on the signal.

Avoid routing analog input signals close to a high-frequency pin (for example, a high-frequency PWM), because the high-frequency switching can be coupled into the analog signal.

If differential mode is used for the ADC12_B, the analog differential input signals must be routed closely together to minimize the effect of noise on the resulting signal.

6.2.2.1 Partial Schematic

Required LCD connections greatly vary by the type of display that is used (static or multiplexed), whether external or internal biasing is used, and also whether the on-chip charge pump is employed. For any display used, there is flexibility as to how the segment (Sx) and common (COMx) signals are connected to the MCU, which (assuming that the correct choices are made) can be advantageous for the PCB layout and for the design of the application software.

Because LCD connections are application specific, it is difficult to provide a single one-fits-all schematic. However, for an example of connecting a 4-mux LCD with 40 segment lines that has a total of 4 × 40 = 160 individually addressable LCD segments to an MSP430FR6989, see the Water Meter Reference Design for Two LC Sensors, Using Extended Scan Interface (ESI).

6.2.2.2 Design Requirements

Due to the flexibility of the LCD_C peripheral module to accommodate various segment-based LCDs, selecting the correct display for the application in combination with determining specific design requirements is often an iterative process. There can be well defined requirements in terms of how many individually addressable LCD segments need to be controlled, what the requirements for LCD contrast are, which device pins are available for LCD use, and which are required by other application functions, and what the power budget is, to name just a few. TI recommends reviewing the LCD_C peripheral module chapter in the MSP430FR58xx, MSP430FR59xx, MSP430FR68xx, and MSP430FR69xx Family User's Guide during the initial design requirements and decision process. Table 6-1 is a brief overview over different choices that can be made and their effects.

6.2.2.3 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, see the LCD_C Controller chapter in the MSP430FR58xx, MSP430FR59xx, MSP430FR68xx, and MSP430FR69xx Family User's Guide.

For additional design information, see Designing With MSP430 and Segment LCDs.

6.2.2.4 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 Water Meter Reference Design for Two LC Sensors, Using Extended Scan Interface (ESI).