Recently, Analog Devices announced two families of single chip energy meter systemon- chip (SOC), ADE71xx/ADE75xx, that may be used as the central unit in smart single phase energy meters. Besides the energy measurement dedicated circuitry, these devices integrate an 8052 microcontroller with Flash memory, an LCD driver, tamper detection circuitry, real-time clock (RTC) and intelligent battery management (Figure 1).
Until recently, all these components were stand alone devices on energy meter boards. Having all of them part of an SOC reduces the overall cost of the board, increases the robustness and the performance of the meter while integrating new features, like the power management, never integrated in an SOC before. The power management circuitry monitors the supply voltage together with the battery and automatically chooses between normal operation mode and an ultra-low power 1.5 µA battery mode. The 8052 microcontroller runs at 4 MHz and its programme is stored in an up to 16 kb of internal Flash memory.
The Flash memory content may be protected against read, writes and erases to increase security and reliability of the solution. The low power RTC enables high accuracy time keeping. It generates special interrupts useful to update the calendar and execute periodical tasks. Tamper detection, a very important task in energy meters, is accomplished by monitoring line and neutral currents and automatically measuring the energy based on the highest current in the system.
Most smart meters have an LCD display used to visualise energy consumption, billing cycle, time, date, RMS voltage and current values. ADE71xx/ADE75xx devices contain an LCD driver capable of driving up to 108 segments. The refresh rate may be adjusted from 128 Hz down to 8 Hz and various degrees of multiplexing (2x, 3x or 4x) and biasing (1/2 or 1/3) are possible. A higher degree of multiplexing and biasing options allows more LCD segments to be driven using the same number of signals. One capability that distinguishes it from other drivers on the market is the contrast control.
As electricity meters are typically placed in an outdoor environment, the temperature of the device may vary wildly from 85°C in the summer to -40°C in the winter. When the temperature lowers, the LCD segments become less and less visible unless the voltage amplitude of the signals driving the segments is increased. ADE71xx/ADE75xx SOCs are ideally suited to manage this phenomenon and maintain the readability of the meter.
They contain a temperature sensor that is used to measure the temperature of the device and a digital to analog converter (DAC) that can vary the voltage amplitude of LCD signals (Figure 2). The DAC is associated to a charge pump circuit, which allows the LCD pins to be driven up to 5 V even if it is supplied at 3.3 V, the voltage supply of ADE71xx/ADE75xx. When the temperature of the SOC is decreasing, the microcontroller embedded in ADE71xx/ADE75xx commands a higher voltage to drive the LCD pins through the DAC and charge pump, maintaining the readability of the display under all conditions.
A table containing the voltages suitable for a particular LCD at various temperatures may be placed in the Flash memory of the SOC. The microcontroller reads the temperature and then, based on the table, commands the right voltage on the LCD display. The charge pump has several other advantages compared to LCD drivers using the resistor ladder biasing technique. The waveforms generated by ADE71xx/ADE75xx SOC have very low offset and very short transition times compared with similar devices that use resistor dividers.
When the charge pump is used 9 mV of offset at room temperature is measured versus 112 mV offset in the resistor divider case of similar bias currents. If the offset is high enough, it may even damage the LCD, so the goal is to drive it using signals that present very low offsets. At minimum, a low offset significantly prolongs the life of the LCD. The charge pump also provides a driving voltage level independent of the power supply level. Another advantage of the LCD driver contained in the ADE71xx/ADE75xx SOC is the capacity to maintain the LCD display in battery mode using very little biasing current – 30 µA. This feature is a critical requirement in a smart meter where, during a power outage, it is required to maintain the LCD to be able to read important consumption and diagnosis information.
The ADE71xx/ADE75xx SOCs have been specifically designed to operate in this situation and have the ability to drive or freeze the LCD segments while consuming very little power. There is also the possibility to blink the display without using extra current.
The management of the contrast in this mode can also be maintained by using a background temperature measurement available in battery mode. If the power outage is long enough, the LCD driver and the charge pump may both be shut down, bringing further savings in power consumption and prolonging battery life. In that moment only the RTC and temperature measurement are still functional and the SOC consumes an industry low 2 µA.
It is common for smart electricity meters to contain a battery. This ensures that the RTC and the microcontroller never lose track of time and date, and data can be stored into the external EEPROM if necessary and the display still functions correctly. ADE71xx/ADE75xx SOCs contain a sophisticated mechanism that monitors the voltages at the input and output of the 3.3 V power supply regulator and the line voltage (Figure 3, VDCIN, VDD, and VP pins).
In this way, it senses immediately when the power of the system begins to collapse and gives time to the microcontroller to, for example, store the state of the meter in the external EEPROM and change the display content while still being supplied from the regular supply. So when the control module switches the supply to the battery, the meter has already finished executing these emergency housekeeping tasks and then goes directly in the sleep mode.
By performing these housekeeping tasks while still operating on the main power supply, the battery life is prolonged. The power management circuitry is used for other tasks, too. The battery voltage is also measured periodically (VBAT input and corresponding ADC in Figure 3) in order to signal when it should be changed. The power management circuitry integrated for this function is sophisticated. It has to switch back and forth between 3.3 V regular supply and 3.6 V of the battery. It has to ensure the supply remains stable during the battery life, allowing the device to function as long as the battery voltage is above 2.5 V.
And it has to allow continuous code execution even while the power supply switches back and forth between the regular supply and the battery. When a power outage is detected, the MCU executes some housekeeping tasks and then the microcontroller commands the device to enter in sleep mode. Only the RTC continues to function, consuming typically only 1.5 µA.
There are moments when the processor needs to wake up: to compensate the RTC for temperature change (temperature measurements are done in the background and do not need MCU actions) and update the charge pump voltage for the LCD. It may also wake up to respond to a communication call using the UART port or to update the calendar. All these events are triggered by internal automatic mechanisms, so the SOC can stay as long as possible in this low power mode. When the power returns to its regular level, the microprocessor is fully operational again after recovering its state from the external EEPROM.
All these operations are executed without the help of any external device, like power on reset (POR), comparators or digital switches, and everything is built inside the SOC. This represents a major cost saving relative to previous generations of electricity meter ICs.
ADE71xx/ADE75xx SOCs are ideally suited to meet the needs of smart electricity meters. They integrate proven energy measurement technology together with a microcontroller, Flash memory, RTC, LCD driver, tamper detection. The power management circuit provides early detection of a power failure. The low power consumption in battery mode enables the meter to function for long periods of time. The LCD contrast control ensures the readability of the display in any conditions.
To facilitate the evaluation of these SOCs and shorten the development of new products, Analog Devices provides a reference design kit that represents an actual energy meter. The package includes full documentation of the board and a software programme that executes basic functionalities that any smart energy meter has to do. It measures energy, manages the LCD display and external EEPROM, communicates externally through a UART port, and supports continuous functionality during power outages. Also included are PC-based programmes that may be used to calibrate and programme the meter in a quasi-industrial environment.