Description

The Wireless M-Bus standard (EN13757-4:2005 and 2012) specifies the RF communication link between water, gas, heat, and electric meters and the data collecting devices and is becoming widely accepted in Europe for smart metering or Advanced Metering Infrastructure (AMI) applications. Wireless M-Bus was originally targeted to operate only in the 868 MHz band, which gives a good trade-off between RF range and antenna size. Recently two new bands (169MHz and 433MHz) have been added to the wM-Bus specification as well, introducing narrow-band solutions with much higher link budget and thus providing longer range solutions than at 868MHz.

What does TI provide to help designers?

Hardware platforms:

• TI offers both single chip (SoC) and two chip solutions for Wireless M-Bus. The two chip solution can consists of ultra-low power MSP430 MCU and a RF device, such as CC110x, CC112x or CC120x for bi-directional metering applications. For lower cost uni-directional wM-Bus enabled products, the CC115L value-line or the high-performance CC1175 transmitter devices are also supported.
• The system-on-chip solution is using the CC430, which integrates a CC1101 radio core and 16-bit MSP430 MCU core.

For new wM-Bus designs the CC112x and CC120x families offer best RF performance with market leading blocking and selectivity performance (see AN121).

Several unique features inside these RF devices enable the market leading RF performance in all wM-Bus applications (Meters or reader units or data collectors):

• Wavematch feature: Advanced Digital Signal Processing for Improved Sync Detect Performance:
  • No false packet detection with 16–bits preamble and 16-bits SYNC words, which are used in N-modes and F-mode
  • Only 4 bits of preamble is sufficient to detect a packet
  • Enables RX sniff mode for very low average RX current without sacrificing RF performance
• ImageExtinctfeature: removes the image component at the system image frequency and eliminates the need for time consuming image calibration steps in during run-time and simplifies system production test
  • Adjacent Channel Selectivity: 65 dB at 12.5 kHz offset
  • Blocking Performance: 90 dB at 10 MHz
  • CC1120 delivers ETSI cat. 1 Compliance in 169 MHz and 433 MHz Bands without external SAW filter (CC1125 adds ETSI Cat.1 performance in the 868MHz band)
• Software solutions:
  • C Source-code (basic PHY layer solution and simple application examples) TI provides software examples to support Wireless M-Bus development, with S- and T-modes implementation as per Application note AN067 (see "Technical documents" below). This is a great starting point for developing own Wireless M-Bus applications and can be easily extended to support other modes as well as complete C source code examples are provided. The code associated with this application note can be run using the EXP430FG4618 board and a CC1101EM-868 module. (CC1101EMK868-915 contains 2 CC1101 EMs and 2 antennas)

  The C-source code for S- and T-modes is now also available for TRXEB + CC1120EM-868MHz. These software examples are not the full wireless M-Bus stack implementation also provided by TI, but demonstrates how the basic transmit, receive and encoding functions can be performed.

  The SmartRF7 Studio Tool together with the TRXEB+CC1120EM (169/433 or 868MHz) can be used as a wM-Bus packet sniffer, displaying the raw packet data as a hexadecimal text string inside the “Packet RX” tab.

• Full wM-Bus Stack for 169, 433 and 868MHz
  • For customers who want to shorten their development cycle and develop metering products, based on a field-proven wM-Bus Stack, TI provides an OMS compliant wM-Bus stack. This is a full SW solution, supporting multiple TI hardware platforms: the two-chip MSP430 + CC110x, CC112x and CC120x radios or the single-chip CC430. Please, contact your local TI representative for more information.

Any particular design challenges to think about?

• For the low level protocol handling, the biggest challenges with the Wireless M-Bus T-mode protocol is the large data rate variation (+/-12% on the Receiver side) as well as the large frequency error (ppm) allowed. In addition, the “legacy” packet format in S- and T-modes is somewhat different compared to recent RF standards as it uses 16-Bytes data blocks followed by 2 Bytes CRC16 fields. Fortunately, the packet engine and demodulator of CC110x and high performance line CC112x or CC120x can handle the wM-Bus packets using the FIFO mode, thus significantly offloading the microcontroller. CC1101, CC112x and CC120x families can handle the +-12% in T-mode reception using the FIFO mode of the Packet engine.
• The CC110x, CC112x or CC120x also support the Manchester encoding and decoding for S-mode in HW, which further reduces MCU code size and complexity.
• The Transceivers can receive data in the FIFO and wake up the MSP430 when a valid packet with matching preamble and SYNC word has been detected. This feature will save power and remove the need to add complex and time critical software that would otherwise be needed to handle up to 100kbit/s over the air data rate and run the preamble and SYNC word detection in SW on the MCU.
• Three different performance classes of transceivers are defined in the Wireless M-Bus standard and the CC110x, CC112x and CC120x comply with the highest performance class Hr for the modes that they support.
• For data security an AES-128 encryption functionality is required. The CC430 and the CC120x family have an AES hardware engine that can handle encryption and decryption in real time. Alternatively, the AES-128 en- and decryption can be handled in SW on the MSP430 in real time.