3. HW architecture of a wM-Bus RF subsystem
A wM-Bus subsystem can be used for both home (HAN) or neighbor area network (NAN) communication, depending on the frequency band or the transmit power used. An example of a HAN implementation is a smart meter with a 868MHz RF link, using either S-, T- or the new C-mode of the wM-Bus protocol. In a real world application, the RF chip (transceiver for bi-directional or transmitter-only device for uni-directional communication) with transmit output power of +10 to +12dBm (measured at the antenna port) and the antenna gain (up to +2dBi) itself will provide the optimum solution within the ETSI 300220 limit of +14dBm EIRP.
The NAN solution for a smart meter will typically be at 169MHz, where the +27dBm EIRP limit enables superior range coverage. Also, the C2-mode (only allowed from data-collector-to-meter) at 869.525 MHz uses a +27dBm limit sub-band and is suitable for NAN applications. For the NAN system solutions, the addition of an external power amplifier is needed, as no integrated RF transceiver chip today can offer +27dBm transmit output power.
The two variants for the hardware of a wM-Bus subsystem, based on the EIRP power limit, are:
Without external power amplifier (blue block in Figure 5
With external power amplifier and optional LNA (e.g. TI’s CC1190 @ 868MHz or dedicated RF front end @169MHz)
Figure 5: Block diagram of a wM-Bus RF subsystem based on TI MCU and RF devices
In fact, the RF sub-system in Figure 5
represents a complete RF module, which is typically connected over UART to the main MCU. In this case, the dedicated MCU (orange block) will run the wM-Bus stack as well as a serial protocol application to connect to the main application MCU.
The second option is to have the wM-Bus stack running on the application MCU and connecting the RF device over SPI interface, avoiding the serial protocol application altogether (orange block in Figure 5
is omitted). It is very important to clarify the hardware and software partitioning inside the smart meter upfront, as there are advantages and disadvantages for both architectures, the main considerations being:
1. Certification of the metrology part and the wM-Bus stack
2. Real-time requirements: both metering and RF communication are time-critical tasks and could sometimes be an issue when the MCU should simultaneously run metrology and communication tasks.
3. Field upgrades of the Firmware: for RF communication and/or the complete Meter.
For those reasons, manufacturers often prefer to keep their systems modular by separating the metrology and the communications functions (a two MCU approach). It can achieve much higher flexibility in the device selection of MCU and RF devices thru independent price and/or performance optimization. Typically, there are multiple pin-compatible MCUs or RF derivatives that offer higher performance or additional features.
The single MCU solution could save some cost, but will usually make the metrology part certification more complex, as the metrology firmware code needs to be secured against tampering or other sources of manipulation or failure.