By Mark Strzegowski
Utilities today are demanding more measurements from their metering endpoints. They also want their meters to contain enough intelligence to implement advanced billing structures, monitor for tampering and more. As soon as the meter contains more than one or two measurements, a liquid crystal display (LCD) is required to display all the information. Time-ofuse or multi-tariff billing drive the need for a real-time clock (RTC) and the rollout of advanced metering infrastructures requires the inclusion of a port to interface to an external communications module.
The inclusion of these features could require up to six separate integrated circuits in a meter. This high component count and associated costs are unacceptable as the features move into mainstream deployment. Integrating the microcontroller (MCU) and peripherals together with the metering front-end has reduced the IC count to only one system on a chip (SOC). The reduction in IC count results in a significant bill of materials (BOM) cost reduction due to three factors: the removal of multiple packages; printed circuit board (PCB) manufacturing yield improvement due to the reduction of solder connections; and overhead reduction by reducing procurement needs.
The single-chip ADE51xx and ADE55xx families of metering devices meet this need for a single chip system by integrating a field-proven metrology core with an 8051 microcontroller, an LCD (liquid crystal display) driver with contrast control, a 100-year full hardware RTC (real-time clock), two UARTs (universal asynchronous receiverstransmitters), and intelligent battery management technology.
The integration provides a unique ability to control the system in ways that were not possible with a multiple IC solution. The ADE51xx and ADE55xx families’ system architecture is designed to minimise current consumption in battery mode while maintaining critical features, like displaying multiple LCD screens, maintaining temperature measurement, and keeping accurate timing during a power outage. All of this circuitry operates in the background so that the MCU core can remain in sleep mode during a power outage.
LARGER AVAILABLE MEMORY SIZE
The ADE51xx and ADE55xx families provide up to 62 Kbytes of on-chip flash memory and 2 Kbytes of RAM (random access memory). None of the flash memory is used to perform the metrology functions and therefore it is all available to the meter designer as program or data memory. The availability of a large memory size is critical to development of a meter which will provide advanced communications, as the implementation of the communications protocols can be very memory intensive. The larger available memory size is also important if the meter design is intended to cover multiple metering requirements for different utilities, or if the programming will be performed in high-level languages, as they tend to generate less optimised code.
BATTERY MODE POWER CONSUMPTION AND SYSTEM ARCHITECTURE
The ADE51xx and ADE55xx families achieve a very low battery mode current consumption of 1.5µA with the system monitoring and RTC active, and 38µA when the LCD display is also turned on. This low current consumption in battery mode enables the meter designer to select a smaller, less expensive battery, which also has a smaller impact on the environment. The benefit of this low current consumption can be lost if the MCU core needs to wake up too frequently to perform housekeeping tasks during a power outage.
The most common housekeeping tasks include adjusting the RTC calibration for temperature variations, or scrolling the LCD display and checking if line power has been restored. The ADE51xx and ADE55xx provide a full hardware RTC with automatic temperature curvature compensation of the crystal oscillator. The system actively monitors the temperature in battery mode and automatically adjusts the curvature compensation of the RTC. This compensation is required to maintain the high accuracy of the on-chip RTC and is performed with no intervention by the MCU, allowing it to remain in sleep mode.
A utility may require that the LCD display scroll through different displays even during a power outage. In some MCU designs this would require powering up the MCU core to load the new patterns to the LCD driver. The LCD driver in the ADE51xx and ADE55xx has four LCD RAM banks built in, that can store up to four separate LCD patterns. The display can then be set to automatically scroll through the patterns without the need of MCU intervention, allowing the MCU core to remain in sleep mode. While in battery mode the ADE51xx and ADE55xx continuously monitor the line power, to see if it has been restored, and will return the MCU and metrology cores to full operation if it has.
The ADE51xx and ADE55xx products differ in that the ADE5166 and ADE5169 include two current channel inputs which are used for anti-tampering detection. The ADE5566 and ADE5569 contain one current channel input for use when the anti-tamper features are not needed. In some countries it is common practice to use two current sensors in a single-phase design to detect tampering, and the ADE51xx family is ideally suited for this type of application. One current sensor is used to detect phase current and the other to monitor the returning neutral current. The ADE51xx parts monitor both of these currents, and if they differ by more than a specified amount a flag will be raised indicating that a fault condition has occurred. The ADE51xx will continue to record the proper energy under earth fault and partial earth fault tampering conditions, by using the larger of the two current channel inputs in the power calculation.
A more sinister form of tampering is the missing neutral tampering condition. In this type of tampering, the neutral is disconnected from the electricity meter. Without a voltage reference, the meter’s board ground floats to the line voltage and the voltage sampling input is invalid.
There are two problems to solve with this fault condition. First a DC supply needs to be developed from a missing neutral power supply, which can be solved by using a transformer to develop an AC voltage from the AC current still flowing in the wire and then turn the AC voltage into a DC voltage with standard power supply techniques. The second problem is that the metering IC needs to detect this fault condition and record the RMS current measurement for future action. The ADE51xx has integrated the ability to switch to a missing neutral mode.
Changes in today’s electricity metering market are driving metering IC providers to integrate more functionality and memory into a single chip. This is being driven by trends to increase the number of measurements, to collect and store more data and to add communication, while maintaining an acceptable cost point. The ADE51xx and ADE55xx product families meet these challenges by providing 4- quadrant measurement capabilities integrated with an MCU core. The MCU core has been optimised for the metering application, with special consideration paid to the battery mode power consumption. With all the changes in the metering industry and the larger energy industry one can only imagine what our children’s electricity meter will look like in the future.