GTEM ESP32 ATM90E26 – Snippets

The ATMEL M90E26 is a high-performance wide-span energy metering chip. The design includes ADC and DSP technologies to ensure the chip’s long-term stability over variations in grid and ambient environmental conditions.

• Power Requirements
  • DC 3.3V ~ 6mA
• Clock
  • 8.192 MHz – External Crystal
• Interfaces
  • SPI (Full Duplex, Synchronous Channel – 3 or 4 wire)
    • 3 Wire. /CS Low
    • 4 Wire. /CS High to Low on each read/write
    • Connected to GTEM ESP32 (GPIO 5/18/19/23)
  • UART (2400 or 9600 Baud)
    • Connected to GTEM ESP32 UART2 (GPIO 16/17)
• Input Voltage Ranges
  • I1 = 120uV to 600mV RMS [When Gain is 1, or 5uV to 25mV RMS when Gain is 24] (GTEM connects to CT Clamp jack socket)
  • I2 = 120uV to 600mV RMS (GTEM Not used but goes to header)
  • VP = 120uV to 600mV RMS (GTEM connects to _ACV_IN_ AC Input via potential divider)
  • VN = 120uV to 600mV RMS (GTEM Not used)
• ADC Resolution
  • 16 bit
• Accuracy
  • Will vary slightly from board to board and setup to setup.  Depends on:
    • ATM90E26
      • Accuracy of 0.1% for active energy and 0.2% for reactive energy over a dynamic
        range of 5000:1, is quoted
      • Electrical parameters measurement: less than ±0.5% fiducial error for Vrms, Irms,
        mean active/ reactive/ apparent power, frequency, power factor and phase angle
      • Phase angle calibration- phase shift
      • In calibration, the only practical method is to adjust for unity power factor with a pure resistive load
        • i.e. No reactive components at all – so no motors and no electronic switch modes etc
        • The reactive power from the M90E26, if there are no phase errors, should read as 0.
          • Reactive power can be considered wasted energy
    • CT Clamp
      • Used to measure AC current.
      • Standard current transformer clamps to sample current, will vary in output slightly
        • Even if moving the CT in different positions of a live cable
      • Not ideal for low current loads
      • On a meter mA range, used to measure the secondary current, the accuracy up to ±(1.5%
        + 5 digits), or between 2.4% and 3.2% depending on the reading.
      • Potential slight phase shift, which may vary depending on current amplitude
      • Low RMS voltages (mV), which can get influenced by noise, at low currents
    • AC (Isolation) Transformer
      • Provides AC waveform reference for accurate Real Power measurements
      • Used to measure AC Voltage
      • Differs from manufacturer to manufacturer
      • Need to ensure transformer does not go into saturation
        • Example flattening seen at the sine waveform peaks
      • Select transformers with cleaner sinewaves
      • If a scope if available, check the ZX (Zero Crossing), output
      • Potential slight phase shift
    • Resistance
      • Accuracy of the connected circuit resistors. e.g. Burden Resistor and potential dividers.
      • Resistors are typically of 1% tolerance.
• Outputs
  • CF1 Active Energy Pulses (Normally Low)
    • Can be used for calibration
    • Reading the RevP/RevQ bit of EnStatus register 0x46
    • Connected to Green LED on GTEM and ESP32 GPIO 34
  • CF2 Re-Active Energy Pulses (Normally Low)
    • Can be used for calibration
    • Reading the RevP/RevQ bit of EnStatus register 0x46
    • Connected to:
      • Amber LED on GTEM
      • GTEM ESP32 GPIO 35
  • IRQ Monitors System Status (Normally Low)
    • Examples, goes High upon
      • Metering / measurement parameter errors
      • Active/reactive energy direction changes
      • Voltage Sag
      • Metering Line Change in Anti-Tamper Mode
    • Max 5mA Load
    • Can be acquired via System Status register 0x01
    • Connected to GTEM ESP32 GPIO 13
  • WO Warning Out (Normally Low)
    • High when Voltage Sag detected via SagWo, register 0x02
    • Fatal Error
    • Max 5mA Load
    • Connected to GTEM ESP32 GPIO 27
  • ZX Zero Crossing
    • 5ms High Level when voltage crosses zero
    • Programmable
    • Max 5mA Load
    • Connected to GTEM ESP32 GPIO 32
• Inputs
  • CS
    • Active Low
  • I1 (P/N)
    • Current Transformer, CT (On GTEM)
  • I2 (P/N)
    • Unused 9 (On GTEM)
  • VN
    • Unused 9 (On GTEM)
  • VP
    • AC Mains Low Voltage Sample
    • Max 600mV RMS via Potential Divider
    • Fed via isolated supply AC Mains transformer
  • Reset
    • M90E26 can be reset when 0x789A is written to software reset register, SoftReset 0x00
  • User Select
    • High = UART Interface
    • Low = SPI Interface
  • Metering Mode
    • MMD0 and MMD1 are used to select the metering mode
    • GTEM solder pad links are provided on the PCB bottom.

• Registers
  • CalStart register 0x20, is 0x6886 after power reset.  It should be set to 0x5678 when calibration is required.
  • CalStart register 0x20, should be set to 0x8765, after calibration.