Stargrid: the new flexible PLC system-onchip platform for smart grids

By Alessandro Moscatelli and Domenico Arrigo

STarGRID™ is a unique flexible, scalable and future-proof power line communication systemon- chip platform ready now to effectively address demanding smart metering and smart grid application requirements.

Today power grids are evolving into smart grids, supported by many international “green” directives and local government initiatives.

By exploiting power grids as the physical medium, power line communication (PLC) is by far the most attractive communication backbone to make networks smarter and stronger.

In particular, STMicroelectronics has pioneered the PLC market since the 1980s, and it has been providing several millions of its most cost effective and integrated state of the art power line transceivers (ST7538 and ST7540) to key meter manufacturers and energy utilities worldwide for large smart metering deployments, such as the ENEL Telegestore and ACEA remote management systems in Italy and the China National Petroleum Corporation sub-metering system in China, as well as other big smart metering systems worldwide, including harsh power grid environments such as South America and Africa.

Along with these field-proven and cost effective solutions, new higher speed PLC technologies offering high communication robustness and future-proof communication services may be required as well in the next years to address new emerging and demanding smart grid applications, while widely accepted open protocols need to be developed and adopted to assure multiple interoperable solutions in order to accelerate wider market penetration.

For these reasons, STMicroelectronics has recently introduced a unique, flexible and scalable PLC system-on-chip (SoC) platform, named STarGRIDTM, which is able to rapidly address all these new evolving smart grid trends.

Today’s PLC solutions for the smart metering market are typically based on hardwired multiple chipsets with fixed and often proprietary modulation schemes and communication protocols, or are based on flexible but expensive and power consuming commercial DSPs with separated additional analog front end (AFE), filtering and power line driver ICs.

The STarGRIDTM platform, instead, offers a perfect tradeoff among high system integration (modem, protocol stack, AFE, filtering and power line driver in a single chip), scalability and flexibility for real cost effective but also fully open and future-proof implementations.

This is achieved by using an advanced STMicroelectronic’s multi-power silicon technology and an innovative and highly flexible system architecture.

The STarGRIDTM modem core comprises an optimised DSP engine with ROM-based program memory able to manage, depending on the embedded firmware, different modulation schemes and physical layer services including performing correction coding and programmable digital filtering. Specific hardware accelerators may be optionally added to the DSP core in some specific STarGRIDTM implementations to address high performing modulations such as OFDM and specific and demanding PHY layer processing, with reduced impact on overall power consumption.

STarGRIDTM system supervising, MAC layer and optional upper protocol layers are managed by an embedded 8-bit 8051 standard industrial micro-core with on-chip dedicated program ROM and data RAM for low cost turnkey protocol implementations either compliant with open standards or with custom specifications.


Figure1 – STarGRIDTM SoC platform block diagram

The possibility to use an external optional Flash program memory for the embedded microcontroller and the integrated JTAG debugger gives the STarGRIDTM platform a higher level of flexibility, allowing the system integrator to develop and debug its own customised protocol stack as well as the possibility to upgrade the protocol during the development stage or for the entire product life, if needed.

The 8051 core comes with standard peripherals such as UART/SPI for external host interfacing, I2C for optional data memory addressing, general purpose I/Os and timers.

Full receiving and transmitting analogue front end (AFE) is also embedded in the platform, including high dynamic range input amplifier with automatic gain control, Sigma-Delta ADC and programmable gain high linear DAC.

Furthermore, the STarGRIDTM platform integrates an ultra linear power line amplifier, with specifically designed topology for on board squeeze size and cost easy programmable filtering, which dramatically reduce application bill of material by eliminating big passive magnetics and caps. The power line amplifier is capable of more than 1 Arms output current and 13 V peak to peak output voltage, which makes it able to drive the most critical real loads, by always keeping the compliance with the stringent CENELEC limits. The requirements set by this standard are not only related to conducted emissions, according to EN50065-1, but also to impedance magnitude limits fixed by EN50065-7 and to immunity against conducted interferences as required by EN50065-2-1.

An intelligent current control loop and thermal monitoring system always guarantee the power amplifier operates in safe conditions delivering the highest performances.

The optional integration of specific functions such as a dedicated hardware accelerator for 128 bit AES data encryption management, a built-in channel quality estimation feature based on signal-to-noise ratio and line impedance measurements, and other critical functions increases system performance and capabilities, while further reducing the total system bill of material and customer development efforts.

By effectively exploiting STarGRIDTM platform scalability and programmability, three main product families have been already derived, named ST757x, ST758x and ST759x, all available in ultra small, pin to pin compatible QFN 48 pin 7×7 mm packages.


Figure 2 – The new Enel-Endesa smart meter for Spanish market equipped with the STarGRIDTM ST758x PLC SoC. The meter also embeds a complete set of semiconductor components from STMicroelectronics

ST7570 provides a full IEC 61334-5-1 compliant PHY and MAC implementation. The device is compatible with IEC 61334-4-32 LLC layer and can be used for standard DLMS/COSEM metering applications or other command and control systems. In particular ST7570 performs a spread frequency shift keying (S-FSK) modulation. The device is able to estimate the signal-to-noise ratio on both channels and to combine that information for automatic best decoding algorithm adoption.

Carrier frequencies and spacing are programmable in CENELEC A, B and C bands with a maximum baud rate of 2.4 kbps. The communication is always synchronised with the 50 Hz/60 Hz main zero crossing, which is automatically detected by the device itself.

Additional protocol services such as alarm messages management and specific networking features supporting repetition with credits for transmission to distant nodes and intelligent enrolment and routing are also embedded.

The ST7580 comes with dedicated turnkey firmware implementing a multi mode n-PSK modulation (B-PSK, QPSK, 8- PSK) with advanced channel quality estimation features (as SNR measurements and mains impedance estimation), dual channel operation and convolutional error coding and Viterbi decoding, achieving extremely robust communication in most all noise conditions including AWGN, narrowband interferes and blockers scenarios. The speed is programmable up to 28.8 kbps as well as carrier frequencies and related spacing can be programmed in CENELEC A, B and C bands. The device is also able to perform standard B-FSK modulation for backward compatibility with ST7538 and ST7540 transceivers.

A light MAC layer is also embedded able to send and receive variable length packets.

The ST758x family has been chosen by the Enel group to provide the new advanced meter management system which will be used by Endesa to install more than 13 million smart electronic meters in Spain. This new solution will use the Enel PLC protocol stack, named SITRED, which is going to be open to the market, so making it available for all the stakeholders who want to adopt a reliable and proven system on more than 50 million meters worldwide.

The ST7590 comes with dedicated turnkey firmware implementing PRIME compliant protocol stack. As one of the first founding members of the PRIME project and Alliance led by Iberdrola, STMicroelectronics has been contributing to PRIME specifications since the beginning and ST7590 is the first full PRIME-compliant SoC suitable for large scale industrial implementations.

The PRIME Physical layer adopts an orthogonal frequency division multiplexing (OFDM) modulation technique working on the CENELEC A band, with 97 sub-carriers using BPSK, QPSK and 8PSK modulation schemes.

Each modulation can be used in combination with a convolutional code to increase the communication robustness obtaining a total of six adaptive different transmitting modes. The maximum data rate is 128.6 kbit/s for the uncoded 8PSK mode.

The PRIME MAC has been conceived to offer a plug and play, reconfigurable communication link, which is able to adapt the sub-network topology to react to the time variability of the power line channel.

The PRIME protocol also specifies the so called convergence sublayer (CS) to interface with existing higher level protocols.

Though there is no practical limitation on the number of Service Specific CSs, two CSs have been defined and specified: the 4-32 CS, which can be used to interface the IEC 61334-4-32 Data Link Layer and the IPv4 CS, which can be used to interface IPv4 protocol. The ST7590 comes with embedded 4-32 CS and IEC 61334-4-32 data link for standard metering applications, as well as IPv4 stack will be addressed.

Following more than 20 years field proven leadership in PLC, a new breakthrough technology from STMicroelectronics, the STarGRIDTM platform, is now industrially ready to effectively and massively address different smart metering and smart grid applications, offering a unique, flexible, scalable fully integrated solution in a single chip.

STarGRIDTM specific turnkey implementations are already available and suitable for major PLC protocols specifications (IEC 61334-5-1, PRIME, SITRED), while new implementations will be supported in the near future to address protocol evolutions of novel regulation proposals to further optimise electricity customer welfare and specific customisations.