Today’s electricity meters have to be designed to offer greater accuracy while using less power and including more features, as well as providing the flexibility to allow the developer to react quickly to changing market conditions. These factors make the microcontroller at the heart of any power meter ever more important.

To address these issues during the development of the H8 Power Meter Pico microcontroller, it was necessary to look at the conflicting demands placed on the next generation of microcontrollers for solid-state electricity meters.

New lower power devices need to offer an increasing number of functions, which can include larger on-chip memory to allow for the more complex software that will be required in future, which will of course be on-chip flash to allow for dynamic reprogramming of the application or configuration data.

These requirements are in many ways diametrically opposed to each other. The need for high levels of integration drive the technology towards advanced semiconductor process technologies to meet the cost targets; however, the use of small geometry processes means leakage and power consumption due to the small cell size will typically increase.

These requirements have resulted in a design based around a multi-chip system-in-package (SiP) device more typically found inside mobile phones to provide the power meter function. 

With the SiP design the functions of the microcontroller are placed on two dies. The first, the CPU die, contains an H8S core along with the on-chip flash and SRAM, timers and analogue-to-digital converter.

The second die is implemented on a special low-power process, and contains all the elements of the design that are required to operate in low power mode. These include features such as a full calibrated real-time clock, LCD driver with booster, comparators, temperature sensor and a watchdog timer.

The analogue die also contains the power supply circuitry required to control the CPU die. This means that to reduce leakage, the CPU die can be completely isolated from Vcc. This feature minimises the current taken when the meter could be kept in “warehouse mode” for many months, or even years, with current consumption down as low as 500µA in some cases. When the power recovers, the CPU die can be awakened by many features, such as an external interrupt, by the comparators or the timers on the analogue chip and Vcc reapplied.

The important function in a power meter is the accurate measurement of voltage and current to calculate the power in real time.

It quickly became clear that there was strong requirement for making the power measurement system as flexible as possible. If a design relies on either hardwired DSP engine functionality or something similar to produce the final result, it can be more difficult to adapt quickly to new market requirements, such as the requirement to measure factors such as power quality.

A typical power meter module provides two independent 16-bit sigma-delta analogue-to-digital converters, each with up to four single-ended or differential analogue inputs and each with its own programmable gain amplifier, one channel for voltage and one for current.

Each channel’s sampling time can be controlled by software, by the use of external pins, or it can be independently controlled by on-chip 16-bit timers, either free running or in synchronisation with the zero crossing of the mains supply. 

This technique allows the phase difference caused by various sensor technologies to be removed in hardware, and this can be programmable if required, depending on the sensor used.

An optional digital integrator is also available on the current channels to support operation with Rogowski coils.

The on-chip data transfer controller (DTC) is designed to automate the process of moving data from the ADC to SRAM, allowing the user to build up buffers representing a half or whole cycle of the mains, which once collected can then be presented to the CPU for processing. 

The DTC is also much quicker at transferring the data. This leaves the CPU available for other housekeeping tasks while the data is being accumulated.

Once the metering data for a complete cycle or half cycle is available in the SRAM, the CPU can be interrupted from its other tasks and can then make the necessary calculations.

The H8 Power Meter Pico has been designed from the bottom up to meet the requirements for the next generation of solid state power meters, and also provides an ideal solution for many other applications. The device is supported by a variety of development tools.

Graeme Clark is product marketing manager at Renesas Technology