Analog front end MCU eases anti-tampering in meter design - Embedded.com

Analog front end MCU eases anti-tampering in meter design

Driven partially by the demand seen most frequently in Asia to separate the analog front end control from the microcontroller used in metering applications, Texas Instruments has developed the MSP430AFE2xx series. The company has also introduced an Energy Library of software tools to ease metering design.

The MSP430AFE2xx series of metrology analog front end (AFE) 16-bit microcontrollers is the first programmable single-phase metrology devices supported with multiple communication interfaces.

The MCUs enable system partitioning in metering applications, such as electricity meters, home automation, sub-metering and energy saving systems.

Based on a 16-bit RISC architecture with a system frequency of 12MHz, offering 3X the system speed over competing parts to drive increased functionality. The microcontroller achieves less than 0.1 percent error in energy accuracy over a dynamic range of 2400:1, enabled by three independent 24-bit sigma-delta converters..

A key part of a meter reading system is a microcontroller that can do the precise measurements, this is the metrology (shown in red below) and is the system that both the customer and the utility provide must rely on to provide accurate readings. “Traditionally there has been a single microcontroller providing the metrology system and what we have started to see, and being driven primarily by the Asian market, is the move towards a two microcontroller based design with a the analog front end (AFE) and a co-processor MCU [labelled metrology MCU],” said Jennifer Barry, MSP430 MCU product marketing at TI.

“The reason for the move to the two-chip solution is to separate the analog functionality from the rest of the system is primarily to isolate from any noise interference. A lot of times the analog-to-digital converters (ADCs) are potentially creating noise as they are directly connected the AC line with quite a lot volatge and current coming in. This is just a way to isolate additional control that atkes palce in the metrology such as controlling an LCD or communicating with external memory or any real time control like keeping date and time,” added Barry.

“The reason that this is being driven by Asia is that there have been stricter regulations on the utility companies and they increasingly adopting this two-chip solution for their meter designs.”

Previously TI has used 16-bit sigma-delta ADCs for metering devices but have incorporated up to three 24-bit sigma deltas with simultaneous sampling for single-phase measurement and anti-tampering. “For single phase metering you use one sigma delta for voltage and another for current, the benefit of the third is when you need to implement anti-tampering by connecting it to a neutral line in the system and if there is any external interference with the meter it sends a spike which can be recorded.”

The simultaneous sampling reduces the amount of software development that would be needed to compensate for the delay in sequential sampling.

Many similar devices are ROM-based  but the inclusion of up to 16KB flash and 512B RAM provides design flexibility with programmable memory via SPI and UART interfaces. Integrated peripherals, including 16-bit timer, watchdog and hardware multiplier allow for customer controlled tasks and math intensive computations in ultra-low power modes.

For the first time TI has assembled in to one energy library a number of software tools for measuring energy, power, voltage, current and several other key metering criteria to provide a robust single-phase metrology implementation. It provides standard C code that can be used with any compiler. It is available as free download and the intention is to expand it with Barry saying the next step is likely to be the addition of LCD control and then wireless would be done in smaller steps as the wireless protocols are adopted by the industry.
 
The MSP430AFE2xx series is also supported by multiple tools, demos and evaluation modules to provide several options for developers to begin evaluation. Allowing users to manage energy usage and begin saving money, the RF-capable MSP430 Energy Watchdog demo displays the electricity consumption of any plug-in appliance on an LCD display.

The programmable MSP430AFE EVM can be used to test the MSP430AFE2xx as a calibrated electricity meter and includes a single-phase electricity meter with sensors and support for RF and infrared, an application note, includes metrology SW and calibration using GUI, while the MSP-TS430PW24 target board and MSP-FET430U24 flash emulation tool can be used to program and debug the MSP430AFE devices.  

The MSP430AFE2xx microcontrollers are priced from $0.80 for 1K units and are immediately available. The MSP-TS430PW24 stand alone target board is priced at $75 while the MSP-FET430U24 flash emulation tool including software and target board is priced at $149.  The MSP430 Energy Watchdog demo and MSP430AFE EVM are available in limited quantities.

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