By Dave Simpson

The past decade has given rise to the most significant change in the global electrical distribution industry. The phasing out of the 1-phase electromechanical meter for its electronic counterpart was made possible with the inherent reliability provided by microelectronic technology.

The driving forces behind the migration away from the 1-phase electromechanical meter since its introduction 120 years ago have been well documented. There was and still remains the ongoing pull from utility companies for increased levels of functionality with its beginnings in prepayment metering, dual tariff systems, and a drive for meter tamper immunity. However the most significant driver for change to electronic meters undoubtedly came in the early-1990s as a result of the ever increasing material cost of electromechanical meter production and the resultant decline in quality and reliability of these meters in some of the world’s largest developing markets.

The first high volume electronic meters produced using standard product metering integrated circuits were simple replacement meters for electromechanical meters, comprising metrology “front-end” integrated circuits measuring active energy and directly driving stepper motor display modules. In parallel, the market requirement for prepayment meters having token credit transfer capabilities expanded, providing the inherent benefits for utility companies and consumers alike.

The introduction of bidirectional energy measurement and then the inclusion of two current inputs in the 1990s signaled the introduction of basic “anti-tamper” meters.

The unsurpassed economic growth over the past decade, the massive expansion of electricity networks particularly in the developing world and the resultant considerable strain put on generation and transmission infrastructure meant that utility companies required a better understanding of customer consumption patterns and an ability to control loads without compromising the service quality level on offer to its customers.

The drive for much higher levels of functionality from the meter itself was inevitable. More sophisticated antitamper features, advanced tariff capabilities and increased measurement parameters, along with the growth in remote meter reading, has given rise to the next leap in meter technology. With all this additional functionality, the expectancy is that meter reliability cannot be compromised. This has been made possible with the introduction of systemon- chip metering technology.

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Figure 1 - An AS8267/AS8268 error curve (the error expressed as a percentage of reading with mains voltage variation)

INTEGRATED FUNCTIONS
The AS8267 and AS8268 system-on-chip 1-phase metering integrated circuits from austriamicrosystems provide all the functional requirements for 1-phase LCD meters. These include a precision 2-current input measurement front-end, an industry standard 8 bit 8051 compatible microcontroller with two universal asynchronous receiver transmitters (UARTs) and a serial peripheral interface (SPI), 32 kB of on-chip Flash memory, a real time clock/calendar, up to 96 segment liquid crystal display (LCD) driver, up to 12 programmable multipurpose inputs/outputs (MPIOs), and an on-chip temperature sensor.

PRECISION MEASUREMENT
The analogue front-end with its 22 bit resolution analogue to digital converters in conjunction with the precision voltage reference provides industry leading accuracy performance of better than 0.1% error over a 2000:1 dynamic range. The consistent repeatability of measurements provided by the AS8267 and AS8268 ICs ensures such accuracy performance is achievable in volume meter production.

The on-chip hardwired signal processor provides for digital phase correction in the two current channels (I1 and I2) and selectable equalisation and high pass filters for attenuation effects and DC offsets respectively. The signal processor also directly generates the energy LED pulse output which precisely represents the measured energy data. This LED pulse output can be used for meter calibration and is fully programmable from 100 to over 200,000 imp/kWh. This allows the option of on-chip digital calibration, providing meter manufacturers with a faster meter production cycle.

The parameters measured by the AS8267 and AS8268 integrated circuits include kWh, Vmains, Imains, kVAr and kVA. The energy, voltage and current measurement calculations are all made in the analogue front-end and signal processor and thus can be digitally calibrated via the onchip calibration registers provided. The dedicated functions provided by the hardwired signal processor provide for a significant reduction in the required program size.

An on-chip real time clock/calendar (RTC) with battery back-up and two alarm registers can be trimmed to a resolution of ±1.4 ppm. The RTC allows for time-of-use and maximum demand measurement, giving the utility company an array of billing options. An on-chip temperature sensor is also provided.

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Figure 2 - Performance of austriamicrosystems’ highly reliable Flash technology and typical alternative technologies available

COMMUNICATION INTERFACES
The AS8267 and AS8268 ICs include two UARTs and a synchronous full-duplex SPI for external communication.

Up to 12 dedicated MPIOs are programmable for data direction, pull-up or pull-down resistors, and drive strength up to 8 mA. This enables direct connection of LEDs to any of the MPIOs for display of reverse current, phase availability and grounding of the load, and all other standard display requirements such as energy LED pulse output for calibration and consumption monitoring. The MPIO can also be set up to accommodate an I2C interface. The fully programmable on-chip LCD driver of up to 96 segments accommodates custom LED displays and can also be used to display special functions such as tamper conditions.

TAMPER FEATURES
The AS8267 and AS8268 ICs provide for all required meter tamper conditions, which have been accommodated in the integrated analogue front-end and digital signal processor.

The two current input channels enable full bidirectional measurement of both the phase and neutral currents, ensuring that energy is measured when tamper conditions are applied, including line in and line out interchanged, phase and neutral interchanged and missing neutral, as well as a combination of these. All tamper conditions can be recorded and displayed with date and time stamps provided by the real time clock.

For conventional single current measurement applications, the second current input channel can be disabled.

RELIABILITY
With this leap in technology now available to the energy metering industry, all would be in vain, without secure program and meter data retention being assured for utility billing purposes.

AS8267 and AS8268 software integrity is provided through the highly reliable austriamicrosystems Flash memory technology. Unsurpassed temperature performance of the on-chip Flash memory operating over a temperature range of -40OC to +125OC ensures over 20 years of data retention even at temperatures as high as +125OC.

Furthermore, the innovative system design allows for security of both programme and data through a combination of password protection and an attack counter. The AS8267 and AS8268 ICs also allow for an additional external EEProm for data storage. The external EEProm is directly accessed by the on-chip 8 bit MCU when connected via the dedicated SPI. Thus none of the available programmable MPIOs are required for this function. The option of connecting an external EEProm using I2C interface can also be implemented using the MPIO.

CONCLUSION
With the system integration provided by the AS8267 and AS8268 ICs comes a considerable reduction in the number of external passive components which can be vulnerable to long term drift. Furthermore, the reduced number of solder joints and the reduced size of the printed circuit board inherently provide for improved system reliability.

The Flash memory performance over the extended temperature range of -40OC to +125OC and the resultant data retention of over 20 years at +125OC provide for unequalled programme and data security, which is adequate for a metering system where the meters operating environmental conditions cannot be guaranteed.

A meter design using the AS8267 or AS8268 integrated circuits provides for a reduced bill of materials (BOM) through the reduced number of components and a more compact design. The end result is a reduction in the meter lifetime cost to the utility companies.

The AS8268 has a dedicated 96 segment LCD driver and 12 programmable inputs/outputs, while the lower cost AS8267 has a dedicated 80 segment LCD driver and 9 programmable inputs/outputs. The AS8267 and AS8268 ICs are pin compatible devices and available in LQFP-64 packages.