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Choosing an MCU for Smart Energy Meters



Smart electricity meters are evolving rapidly with different architectures used in markets around the world (as well as different regulatory requirements). Since they are in the process of being rolled out to utility customers by the hundreds of millions, there is great interest in — and great reward for — successful smart meter designs.

In their most basic form, utility meters provide energy and power measurement, data transmission, real-time clock upkeep, and data display on the meter’s front panel. Key design requirements for smart meters include the following: (1) they should operate at low energy so they can run for prolonged periods on battery power, and (2) they must include security features that can protect the content of communications and the safety of stored data.

Basic meters also provide one-way communication, enabling electricity providers to read the meter automatically and remotely using different communication solutions including RF wireless, power line carrier, and General Packet Radio System (GPRS) data communications.

Smart meters with Advanced Metering Infrastructure (AMI) architecture provide two-way communications and offer the benefits of improved reliability and accuracy, the ability to monitor outages, and provide remote disconnect as well as the option of adding variable tariffs laden with incentives to consumers to shift peak loads. Smart meters can also communicate directly with other meters and with in-house display units to allow both the utilities and their customers to better manage energy consumption.

As implementations and architectures become more sophisticated, electricity meters demand more processing power and more flash memory for software stacks, communication protocols, and firmware updates. The meter also has a communications interface. In the U.S., many companies have selected the ZigBee wireless radio as the link to the utility, while in Europe, a number of utility groups have agreed to use power-line communications nodes.

MCU requirements

Low power consumption is a principal requirement of the smart energy meter and, in turn, of the MCU that enables sensing/measuring of power usage. Low power consumption is also beneficial because even though electricity meters are powered by the mains, they must be able to use battery power if power is lost so the real-time clock (RTC) remains running.

MCUs for a smart meter application need to have high-resolution A/D converters for current and voltage measurement; usually 16- or 24-bit A/D speed is not an issue, so sigma-delta converters may be used. Dual A/Ds are usually needed for simultaneous measurements and a third A/D may be needed for temperature measurement and intrusion detection — a must to prevent meter tampering. The data transmission will most likely need to be encrypted using AES, DES, RSA, ECC, or SHA-256. An IC with high EMC rejection reduces the need for external components. And an EEPROM may be required for data logging and to store calibration data.

Metrology could be a one-, two-, or three-phase energy measurement. A single-phase meter is common in most residential applications. This typically has one voltage and one current to be measured, and it supports low to medium load. A dual-phase meter, which is not as common worldwide and is employed mainly in Japan, has two voltages and two currents to be measured. Each phase is off by 180º, and it is typically for medium to large loads. Finally, three-phase measurements are commonly used for large office spaces and industrial applications. There are three different phases that are 120º out of phase with each other. Three voltages and three currents need to be measured, so a minimum of six ADCs are needed to get an instantaneous snapshot of energy consumption and power factor. Inclusion of a programmable gain stage for each A/D in the candidate MCU is a big aid to the sensor interface.

In energy-metering service, an MCU may have to handle many things. Figure 1 is a functional block diagram that shows the processor in the center and also the various peripherals the processor may be required to handle in a good smart meter design.

Figure 1: A typical smart meter block diagram.

So now that we have defined the requirements of an MCU for smart energy meter service, where do we find such a thing? Here are a few possibilities.

32-bit energy-metering IC

The NXP EM773FHN33 is an ARM Cortex-M0 based, low-cost, 32-bit, energy-metering IC. It runs at 48 MHz and features a nested vectored interrupt controller, serial wire debug, 32 Kbytes of flash, and 8 Kbytes of SRAM. Also, in its peripheral complement, the MCU includes an I²C bus interface, an RS-485/EIA-485 UART, one SPI interface with SSP features, three general-purpose counter/timers, up to 25 general-purpose I/O pins, and a “metrology engine” designed to collect voltage and current inputs to calculate the active power, reactive power, apparent power, and power factor of a load.

There are two current inputs and a voltage input, and the part has a stated 1 percent measurement accuracy. It comes in a 7 x 7 x 0.85 mm HVQFN plastic thermally enhanced, thin quad flat package with 33 terminals. The energy-metering IC is 1 percent accurate for scalable input sources up to 230 V/50 Hz/16 A and 110 V/60 Hz/20.

16-bit MCU with high-resolution ADCs

The Texas Instruments MSP430AFE253IPW low-power 16-bit MCU targets utility metering applications with a single-phase metrology analog front end that supports 0.1 percent accuracy over a 2,400:1 dynamic range. The MSP430AFE253IPW has three 24-bit sigma-delta A/D converters and up to 16 Kbytes of flash, 512 bytes of RAM, and temperature measurement.

This MCU also has one, faster, 10-bit A/D. The accuracy spec given for the 24 bit A/D is an offset error of ±0.2 percent of FS maximum – which makes it about a 19 bit converter. Active mode supply current is only 220 µA at 1 MHz, 2.2 V and standby is 0.5 µA. It runs at -40° to 85°C. One of the A/Ds can be used for tamper protection.

There are nine versions of the MSP430AFE2xx device family (Figure 2), and all have SPI and UART interfaces, an LCD controller, 16 bit timers/PWMs, a watchdog, and a hardware multiplier. These chips do not have a real-time clock or data encryption.

Figure 2: TI’s MSP430AFE2xx family offers SPI and UART interfaces and an LCD controller.

8- or 32-bit options

The 8-bit Freescale MC9S08GW MCU (Figure 3) features two 16-bit A/D converters with dedicated differential amplifiers and up to 16-channels. The device has 64 Kbytes of flash, an RTC with tamper protection, a LCD controller for up to 288 segments, and CRC data checking. It runs at up to 20 MHz at 3.6 to 2.15 V and up to 10 MHz at 1.8 V. The chip comes in a 10 x 10 mm or 14 x 14 mm LQFP package.

Another possibility from Freescale is their K30 Cortex-M4-based 32-bit MCUs with a low-power segment LCD controller that drives up to 320 segments (Figure 3). The PK30X256VLQ100 has a single 6-bit A/D converter, 256 Kbytes of flash, a RTC, an interrupt controller, and CRC data checking.

Figure 3: Freescale K30 block diagram.

MCUs with an LCD driver and low-power modes

Microchip’s PIC18F87K90 is a good choice for metering, though its 24-channel A/D conversion is limited to 12-bit resolution. It has a real-time clock, 128 Kbytes of flash and 1 Kbyte of EEPROM, plus LCD drive for 192 pixels and four external interrupts. In power down mode, the IC’s supply current is a maximum of 600 nA at 60°C. The RTC takes a maximum of 4.6 μA at 3.3 V and 60°C. The A/D integral linearity error is ±1 LSB typical, but is ±6.0 LSB maximum — quite a spread. The differential linearity error is specified as ±1 typical and +3.0/–1.0 maximum. This is over the industrial temperature range. No encryption or tamper proofing is provided.

An SoC approach

A somewhat different approach is taken by Analog Devices, whose ADE7880 isn’t really an MCU but more of an SOC with “computational blocks” tuned for the electronic-meter application. It is intended for three-phase energy measurement and features an adaptive real time monitoring harmonic engine.

The ADE7880 device incorporates second-order sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), a digital integrator, reference circuitry, and all of the signal processing required to perform the total (fundamental and harmonic) active, and apparent energy measurements, rms calculations, as well as fundamental-only active and reactive energy measurements The IC can monitor three user-selectable harmonics, in addition to the fundamental. It automatically tracks fundamental frequency and provides real-time harmonic measurement updates. Harmonic analysis includes current rms, voltage rms, and active, reactive, and apparent powers, power factor, plus harmonic distortion, and total harmonic distortion plus noise (THD + N) calculations.

The ADE7880 incorporates seven second-order Σ-Δ A/D converters, a digital integrator, reference circuitry, and all the signal-processing power required. It supports IEC 62053-21, IEC 62053-22, IEC 62053-23, EN 50470-1, EN 50470-3, ANSI C12.20, and IEC 61000-4-7 standards and takes about 25 mA to operate.

Summary

The six example MCUs for smart meter applications discussed here are very capable and form the focal point of a smart meter system. While some MCUs are available with integrated AFEs, in other cases signal capture and conversion requirements can lead to the use of a separate analog-front-end chip. In an electricity meter, the AFE senses the current and voltage, converts the sensed values into digital form, and then sends the digital values to the microcontroller. In all cases, other components will be needed for full smart meter operation. Peripheral devices essential for smart meters are devices such as EEPROM chips and a photocoupler that provides line isolation. And, of course, software is needed to carry out diverse data-processing functions—including power usage calculations, and processing of a customer’s energy costs.

That said, all of the MCUs mentioned are available now, and with one or two external ICs the complete smart meter function can be implemented — and at very low power consumption. For more information, use the links provided to access product pages on the Hotenda website.
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