A modern digital meter looks simple from the outside but contains several tightly integrated subsystems. This page walks through each block and explains why it exists.

The Big Picture

Measurement Front-End

This is the analog section that converts high-voltage mains signals into small, safe signals the IC can read.

Voltage sensing uses a precision resistor divider to scale 230 V down to millivolts. Current sensing uses either a shunt resistor (direct, lossy) or a Rogowski coil / CT (isolated, preferred for safety).

define CSENSOR annotation=U label="Current Sensor" {
  pin 1 INP left
  pin 2 INN left
  pin 3 OUT right
  pin 4 VCC top
  pin 5 GND bottom
}

define MET_IC annotation=U label="Metrology IC" {
  pin 1 Iin left
  pin 2 Vin left
  pin 3 VCC top
  pin 4 GND bottom
  pin 5 SPI right
}

define MICROCTRL annotation=U label="MCU" {
  pin 1 SPI left
  pin 2 VCC top
  pin 3 GND bottom
}

define CAP annotation=C label="Bypass" {
  pin 1 P top
  pin 2 N bottom
}

U1: CSENSOR value="INA282"
U2: MET_IC value="ADE7758"
U3: MICROCTRL value="STM32"
C1: CAP value="100nF"

layout direction=LR gap=60

U1.INP --> global:LINE
U2.Vin --> global:LINE
U1.INN --> global:GND
U1.OUT --> U2.Iin
U2.SPI --> U3.SPI
U1.VCC --> global:VCC
U2.VCC --> global:VCC
U3.VCC --> global:VCC
C1.P --> global:VCC
U1.GND --> global:GND
U2.GND --> global:GND
U3.GND --> global:GND
C1.N --> global:GND

Metrology IC

The metrology IC is the heart of the meter. It:

• Digitises voltage and current simultaneously
• Computes active, reactive, and apparent power
• Accumulates energy in pulse-output registers
• Measures frequency, RMS voltage, RMS current, and power factor
• Reports all values to the MCU over SPI or I²C

Popular ICs: ADE7758 (Analog Devices), CS5480 (Cirrus Logic), ATM90E36 (Microchip).

MCU

The MCU reads energy registers from the metrology IC, manages the RTC, drives the display, handles tariff logic, stores data to flash, and runs the communication stack. It is typically an ARM Cortex-M3 or M4 running at 48–120 MHz.

The MCU also runs the DLMS/COSEM stack — the communication protocol that utilities use to read meters remotely.

Real-Time Clock (RTC)

The RTC tracks wall-clock time independent of the MCU. It has its own battery backup so time is maintained even during a power outage. Accurate time is critical for:

• Time-of-use (ToU) tariff switching
• Load profile timestamps
• Event log timestamps
• Billing period boundaries

Non-Volatile Memory

Meters store years of historical data. Typical memory layout:

Communication Module

This is the pluggable or soldered module that connects the meter to the outside world. A single meter design may support multiple communication options via interchangeable modules:

• Optical port — local readout via infrared (IEC 62056-21)
• RS-485 — wired bus to a data concentrator
• PLC — data over the power line itself (no extra wiring)
• RF — wireless mesh network
• GPRS / 4G / NB-IoT — direct cellular backhaul

Power Supply

Meters operate from the mains supply they are measuring. An internal SMPS converts 230 V AC to the 3.3 V / 5 V DC rails needed by the electronics. The SMPS must work across a wide voltage range (70–270 V AC) to function during grid brownouts.

Key Takeaway

Every block in a meter exists for a specific reason tied to billing accuracy, data integrity, or communication reliability. When a meter fails in the field, the failure usually traces back to one of these blocks — most commonly the power supply or the communication module.