SLAA517F May 2012 – August 2021 MSP430F6720A , MSP430F6720A , MSP430F6721A , MSP430F6721A , MSP430F6723A , MSP430F6723A , MSP430F6724A , MSP430F6724A , MSP430F6725A , MSP430F6725A , MSP430F6726A , MSP430F6726A , MSP430F6730A , MSP430F6730A , MSP430F6731A , MSP430F6731A , MSP430F6733A , MSP430F6733A , MSP430F6734A , MSP430F6734A , MSP430F6735A , MSP430F6735A , MSP430F6736 , MSP430F6736 , MSP430F6736A , MSP430F6736A
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The MSP430F6736(A) device is the latest metering system-on-chip (SoC), that belongs to the MSP430F67xx family of devices. This family of devices belongs to the powerful 16-bit MSP430F6xxx platform and includes new features and flexibility to support robust 1-phase, 2-phase, and 3-phase metrology solutions. This application report, however, discusses the implementation of a 1-phase solution only. These devices find their application in energy measurement and have the necessary architecture to support them.
The MSP430F6736(A) has a powerful 25-MHz CPU with MSP430CPUX architecture. The analog front end consists of up to three 24-bit ΣΔ analog-to-digital converters (ADC) based on a second-order sigma-delta architecture that supports differential inputs. The sigma-delta ADCs (ΣΔ24) operate independently and can output 24-bit results. They can be grouped together for simultaneous sampling of voltage and currents on the same trigger. In addition, the ADCs have an integrated gain stage that supports gains up to 128 for amplification of low-output sensors. A 32-bit x 32-bit hardware multiplier on this chip can be used to further accelerate math-intensive operations during energy computation. The software supports calculation of various parameters for 1-phase energy measurement. The key parameters calculated during energy measurements are: RMS current and voltage, active and reactive power, active and reactive energy, power factor, and frequency.
The MSP430F6736(A) source code along with the EVM430-F6736 design files that are described in this application report can be downloaded from the One-Phase Metrology with Enhanced ESD Protection Reference Design.
Figure 2-1 shows typical connections of electronic electricity meters (also called energy meters or e-meters) in real-life applications. The supported ac voltages and associated currents are 230 V or 120 V at 50 Hz or 60 Hz. The labels LINE and NEUTRAL indicate low-voltage ac coming from the utilities.
More information on the current and voltage sensors, ADCs, and other hardware is provided in the following sections.
Figure 2-2 shows a block diagram of the high-level interface that is used for a single-phase energy meter application using the MSP430F6736(A). A single-phase two-wire star connection to the mains is shown with tamper detection. Current sensors are connected to each of the current channels, and a simple voltage divider is used to measure the corresponding voltages. The CT has an associated burden resistor that must be connected at all times to protect the measuring device. The choice of the CT and the burden resistor is done based on the manufacturer and current range required for energy measurements. The choice of the shunt resistor value is determined by the current range, gain settings of the SD24, and the power dissipation at the sensors. The voltage divider resistors for the voltage channel are selected to make sure that the mains voltage is divided down to the normal input ranges that are valid for the MSP430™ SD24. For these values, see the MSP430F5xx and MSP430F6xx Family User's Guide and the device-specific data sheet (MSP430F673x, MSP430F672x Mixed-Signal Microcontrollers and MSP430F673xA, MSP430F672xA Mixed-Signal Microcontrollers).
L and N refer to the line and neutral voltages, respectively, and are interchangeable as long as the device is subject to only one voltage and not both simultaneously at its pins. The other signals of interest are the PULSE1 and PULSE2. They are used to transmit active and reactive energy pulses used for accuracy measurement and calibration.