Microcontroller for power meters thwarts tampering
By Margery Conner, Technical Editor -- EDN, October 6, 2009
Government agencies and utilities are pushing to move the global power infrastructure to a smart grid—a transition that will rely on the installation of intelligent power meters in residences and businesses. Addressing that need, Freescale has introduced the MCF51EM microcontroller family for metering, including algorithms, tamper resistance, the ability to update without powering down, real-time-clock independence, and self-validation. Freescale based the family on the ColdFire microcontroller core. It includes code for calculating active, reactive, and apparent energy that meets IEC (International Electrotechnical Commission) 62053-22 International Energy Metering Specification Class 0.5 accuracy readings for active energy.
The part’s tamper-detection circuitry and firmware take data from sensors or mechanical switches that monitor physical intrusion or damage to a meter’s case. The tamper-detection signal causes the chip’s independent real-time clock to log the time and date of the tampering for reporting to the utility. The device also logs attempts to manipulate the meter’s power rails, during which the meter can operate from a battery backup to log tampering incidents. The independent real-time clock also has 32 bytes of protected RAM that the clock’s battery backup maintains.
The chip supports secure software updating in the field without powering down or rebooting with its segmented 128- to 256-kbyte flash memory. For example, a typical metering application might fit into 128 kbytes of RAM. The processor downloads the new software and builds a duplicate copy of the current software in the other 128 kbytes and then hot-swaps the new code after it has validated the code with a hardware-based CRC (cyclic-redundancy-check) test.
Rather than relying on software calculation of the phase lag between voltage and current, the part has four 16-bit SAR (successive-approximation-register) ADCs that feed into a hardware sequencer for fast, accurate phase measurements. The chip also includes an optoisolator interface for maintenance personnel’s safety. A reference design employing the MCF51EM256 version implements a single- or three-phase smart electrical meter and sells for $3.60 (10,000).
