Reference design and specialized silicon make it easy to design cost-effective smart electrical meters for single- and three-phase applications.

With STMicro, ADI and several other competitors to contend with, Freescale was a little late off the block in tailoring its microcontrollers to address the specific needs of the Smart Meter market - but their MCF51EM MCU family of controllers puts them squarely back into the game. Among other application-specific features, this version of their 32-bit ColdFire core-based processor includes high-precision analog front end metrology, a real-time clock, and an LCD controller. When combined with a complete suite of reference software, you get a solution that should dramatically slash both development and BOM costs.

The ‘51EM’s crown jewel is its four channel 16-bit (13.5 bit effective resolution) SAR ADC array which enables the MCU to support the voltage and current monitoring functions for a single-or three-phase meter  using only a handful of passive components. All ADC channels sport a four-channel mux, enabling a single converter to handle current measurements for all three phases with some capacity left over for housekeeping functions. Freescale says the converters are controlled by an on-chip programmable sequencer that can be programmed for whatever delay you want to use between voltage and current measurements without having to make the MCU to do it in software.  

It might seem a bit like overkill to equip a humble power meter controller with a 32-bit processor but I think Freescale made the right call here. For today’s applications, it will make quick work the more complex calculations and algorithms involved with high-precision power measurements (more on this shortly) with enough extra processing power to support today’s automated meter reading (AMR) functions as well as the advanced metering infrastructure (AMI) protocols that will be used support Smart Grid applications such as rate negotiation and rate-based 2-way power billing. The current reference design supports 0.5% accuracy (worst case) – significantly better than one can get from most electromechanical systems. While 0.5% should meet most global accuracy standards for the immediate future, Freescale is already working on a follow-on reference design and software package that should deliver 0.2% accuracy.

Freescale has made lots of other small improvements to the ‘51EM family to give it increased functionality and reliability. For example, its memory is fabricated using a so-called “robust flash” (128 o 256k) which includes hardware a CRC function to insure security and firmware integrity. The memory is split into two swappable banks, enabling on-the-fly firmware upgrades. Like most other power meter chips on the market the controller has some hardware-based tamper detect capabilities. In this case Freescale has equipped the real-time clock with a circuit that that senses battery power interruption – a common method employed by would-be power thieves to disable or confuse a meter. If the battery is interrupted the clock circuit sets a flag to alert the processor so it can log the event and, if it’s capable of doing so, alert the utility company of the incident. The tamper-detect circuitry also locks the real time clock’s calendar until an unlock code is provided.

Another feature worth noting is the processor’s serial interfaces (and drivers) that support for communication with the outside world. The reference design is intended to interface to 3^rd-party grid communications solutions including powerline modems and wireless links. Freescale says they plan to offer lots of other communication solutions in the near future, through internally-developed products, partnerships, or acquisitions. One interesting candidate is On’s recently-announced powerline modem chip which I featured in this week’s NewsBites review.

Developers will find an exceptionally complete package of software modules that includes everything from a user interface to a calculation/measurement module that controls the analog converter system and performs energy use calculations. The modules are packaged as part of a near-turnkey application that fits easily into a 128k chunk of flash. The package can be customized using Freescale’s Code Warrior enhanced IDE or their Processor Expert graphical programming tool. According to Freecale, Processor Expert sits on top of Code Warrior and enables hassle-free, code-free customization of their existing code.

The hard-core programming community that prefers “rolling their own” code can take comfort in the fact that Freescale will make the MQX RTOS and its associated middleware available for this chip in December. While not as critical for the current well-defined AMR applications, making an RTOS available is critical for the development of more advanced, less well-defined functions. I expect the RTOS will be most valuable in Smart Grid applications that require the processor to split its time between comms and metrology tasks without missing a beat. It goes without saying that the RTOS and its associated tools will be able to re-use any of Freescale’s existing software modules

On the hardware side, the reference design is supported by both a development platform and production-ready PCB designs. If you wish, you can even use Freescale’s royalty-free case design.

The smart power meters that devices like Freescale’s controller makes possible will play an essential role in helping utilities support advanced load management, distributed generation, and make renewable energy sources a significant fraction of their overall generation portfolio. Freescale is also looking towards even more advanced applications and engaging with both internal and external partners on smart building and vehicle-to-grid (V2G) applications. Many of these applications are a good fit for Freescale’s current 802.15 solutions  although I would expect that at least some of the MAC functionality will eventually find its way onto the controller chip.  I’d expect to see some interesting developments on some of these more advanced applications as standards and markets mature – perhaps even within the next 2 quarters.

The MCF51EM256/128 devices are currently available for limited sampling, with broad availability expected in the first quarter of 2010. Suggested resale pricing will begin at $3.60 for quantities of 10,000 units.

The polyphase reference design is available now for loan and evaluation through local Freescale sales representatives. Technical documentation is now available for both the reference design and the MCF51EM are available from the Freescale web site by clicking here.

To see Freescale’s announcement of their smart meter reference design click here.