TIDUEM7A April   2019  – February 2021

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 End Equipment
      1. 1.1.1 Electricity Meter
    2. 1.2 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 ADS131M04
      2. 2.2.2 TPS7A78
      3. 2.2.3 MSP432P4111
      4. 2.2.4 TPS3840
      5. 2.2.5 THVD1500
      6. 2.2.6 ISO7731B
      7. 2.2.7 TRS3232E
      8. 2.2.8 TPS709
      9. 2.2.9 ISO7720
    3. 2.3 Design Considerations
      1. 2.3.1 Design Hardware Implementation
        1. 2.3.1.1 TPS7A78 Cap-Drop Supply
        2. 2.3.1.2 TPS3840 SVS
        3. 2.3.1.3 Analog Inputs
          1. 2.3.1.3.1 Voltage Measurement Analog Front End
          2. 2.3.1.3.2 Current Measurement Analog Front End
      2. 2.3.2 Current-Detection Mode
        1. 2.3.2.1 ADS131M04 Current-Detection Procedure
        2. 2.3.2.2 Using an MCU to Trigger Current-Detection Mode
          1. 2.3.2.2.1 Using a Timer to Trigger Current-Detection Mode Regularly
          2. 2.3.2.2.2 MCU Procedure for Entering and Exiting Current-Detection Mode
        3. 2.3.2.3 How to Implement Software for Metrology Testing
          1. 2.3.2.3.1 Setup
            1. 2.3.2.3.1.1 Clock
            2. 2.3.2.3.1.2 Port Map
            3. 2.3.2.3.1.3 UART Setup for GUI Communication
            4. 2.3.2.3.1.4 Real-Time Clock (RTC)
            5. 2.3.2.3.1.5 LCD Controller
            6. 2.3.2.3.1.6 Direct Memory Access (DMA)
            7. 2.3.2.3.1.7 ADC Setup
          2. 2.3.2.3.2 Foreground Process
            1. 2.3.2.3.2.1 Formulas
          3. 2.3.2.3.3 Background Process
            1. 2.3.2.3.3.1 per_sample_dsp()
              1. 2.3.2.3.3.1.1 Voltage and Current Signals
              2. 2.3.2.3.3.1.2 Frequency Measurement and Cycle Tracking
            2. 2.3.2.3.3.2 LED Pulse Generation
            3. 2.3.2.3.3.3 Phase Compensation
    4. 2.4 Hardware, Software, Testing Requirements, and Test Results
      1. 2.4.1 Required Hardware and Software
        1. 2.4.1.1 Cautions and Warnings
        2. 2.4.1.2 Hardware
          1. 2.4.1.2.1 Connections to the Test Setup
          2. 2.4.1.2.2 Power Supply Options and Jumper Settings
        3. 2.4.1.3 Software
      2. 2.4.2 Testing and Results
        1. 2.4.2.1 Test Setup
          1. 2.4.2.1.1 SVS and Cap-Drop Functionality Testing
          2. 2.4.2.1.2 Electricity Meter Metrology Accuracy Testing
          3. 2.4.2.1.3 Current-Detection Mode Testing
          4. 2.4.2.1.4 Viewing Metrology Readings and Calibration
            1. 2.4.2.1.4.1 Viewing Results From LCD
            2. 2.4.2.1.4.2 Calibrating and Viewing Results From PC
              1. 2.4.2.1.4.2.1 Viewing Results
              2. 2.4.2.1.4.2.2 Calibration
                1. 2.4.2.1.4.2.2.1 Gain Calibration
                  1. 4.2.1.4.2.2.1.1 Voltage and Current Gain Calibration
                  2. 4.2.1.4.2.2.1.2 Active Power Gain Calibration
                2. 2.4.2.1.4.2.2.2 Offset Calibration
                3. 2.4.2.1.4.2.2.3 Phase Calibration
        2. 2.4.2.2 Test Results
          1. 2.4.2.2.1 SVS and TPS7A78 Functionality Testing Results
          2. 2.4.2.2.2 Electricity Meter Metrology Accuracy Results
          3. 2.4.2.2.3 Current-Detection Mode Results
  8. 3Design Files
    1. 3.1 Schematics
    2. 3.2 Bill of Materials
    3. 3.3 PCB Layout Recommendations
      1. 3.3.1 Layout Prints
    4. 3.4 Altium Project
    5. 3.5 Gerber Files
    6. 3.6 Assembly Drawings
  9. 4Related Documentation
    1. 4.1 Trademarks
  10. 5About the Author
  11. 6Revision History
2.3.2.3.3.1.2 Frequency Measurement and Cycle Tracking

The instantaneous voltages, currents, active powers, and reactive powers are accumulated in 64-bit registers. A cycle tracking counter and sample counter keep track of the number of samples accumulated. When samples of approximately one second have been accumulated, the background process stores these accumulation registers and notifies the foreground process to produce the average results, such as RMS and power values. Cycle boundaries are used to trigger the foreground averaging process because this process produces very stable results.

For frequency measurements, a straight line interpolation is used between the zero crossing voltage samples. Figure 2-20 shows the samples near a zero cross and the process of linear interpolation.

GUID-3B4903C0-2547-44B6-BA2D-3289BAC32BD4-low.jpgFigure 2-20 Frequency Measurement

Because noise spikes can also cause errors, the application uses a rate-of-change check to filter out the possible erroneous signals and make sure that the two points are interpolated from genuine zero crossing points. For example, with two negative samples, a noise spike can make one of the samples positive, thereby making the negative and positive pair appear as if there is a zero crossing.

The resultant cycle-to-cycle timing goes through a weak low-pass filter to further smooth out any cycle-to-cycle variations. This filtering results in a stable and accurate frequency measurement that is tolerant of noise.