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Simpler Architecture Offloads MCU in 2.4 GHz Wireless Applications

Tue, 05/29/2012 - 11:36am
Prashant Dekate, ams AG

In more and more consumer and industrial applications, end users appreciate the convenience of connectivity without cables, and the license-free 2.4 GHz ISM band is often the most suitable choice. Such applications include body area networks, wireless sensor networks, active smart labels, home/building automation, and interactive remote controls.
But the effect of so many 2.4 GHz devices on the market is that the spectrum has become crowded.

One of the attractions of a well-known wireless standard such as Bluetooth or ZigBee is that their protocols are robust enough to overcome the interference that 2.4 GHz transceivers encounter. Using a standard protocol such as these is certainly a more attractive option for device manufacturers than developing a custom protocol from scratch, which requires deep knowledge of RF operation and is a huge undertaking.

But the reality is that protocols such as Bluetooth and ZigBee have both the advantage and disadvantage of a standard. They are feature-rich, which means they are applicable to a very wide range of applications, but this makes them over-specified for simpler applications. So the microcontroller (MCU), which is normally intended for other functions, has to deal with the heavy overhead of running a large protocol software stack. This has a detrimental effect both on speed of operation and power consumption. The license fee paid by developers of standards-based systems can also make a marked addition to bill of materials costs.

Industry-standard protocols are often inappropriate or uneconomic for embedded systems that have to be simple and quick to develop, but which require interference immunity in the 2.4GHz frequency band. For such 2.4 GHz wireless networks, austriamicrosystems’ AS3940 transceiver takes a different architectural approach.

Protocol management integrated into the transceiver
This new architecture removes the protocol from the MCU and locates it instead in a block called the ‘Link Manager’ (see Figure 1). With a simple Serial Peripheral Interface (SPI) connection to the MCU, the developer now has a minimal set of commands to execute at the MCU in order to configure and control the radio network.

 

Fig. 1: transceiver with Link Manager removes overhead from the MCU

This link manager implements all of the most complex elements of the protocol stack, including:

  • Pairing and wake-up procedures
  • Automatic data transfer (in burst or streaming mode)
  • Timings and frequency channels to be used for any transmit/receive instructions
  • Retransmissions and acknowledgements
  • Channel hopping
  • Forcing the transceiver in the different power modes.
  • Modulation schemes
  • SPI interface

Because these protocol functions are implemented in dedicated circuitry, this architecture can provide power consumption and speed improvements over an architecture in which protocol code is executed in a general-purpose microcontroller.  An added advantage is that the same MCU, which is used in the older system, can be reused with minimal overhead.

The AS3940 from ams is a 2.4 GHz multi-channel Frequency Shift Keying (FSK) transceiver that implements this Link Manager concept. Its proprietary protocol allows it to configure itself based on a set of 158 registers and the 13 commands shown in Figure 2.

do_pair

do_mcdt_wakeup

do_temp_pair

do_mcdt_tx

do_wakeup

do_mcdt_rx

do_wakeup_from_client

do_mcdt_standby

do_wakeup_from_master

do_mcdt_sleep

do_send_lib

do_sw_reset

Force_standby

 

Figure 2: the Link Manager can be controlled with just 13 commands

This small set of commands can be implemented very quickly even in a simple microcontroller. As a result, development projects that would take months using a standard protocol such as Bluetooth or ZigBee can be completed in weeks with a transceiver that integrates a Link Manager (see Figure 3).

 

Fig. 3: AS3940 2.4 GHz star network transceiver block diagram

So reducing complexity and development time have been adequately provided for: the AS3940 transceiver-plus-Link Manager concept is simple to implement and reduces overhead on the MCU.

But can this simple implementation at the same time provide sufficient features to support typical application requirements? And can it provide the interference immunity required for a radio operating in the crowded 2.4 GHz spectrum?

Data rate and network topology
The Link Manager of the AS3940 can be used to easily set up a permanent client-master link which can transmit data in short bursts with programmed intervals, or a streaming link with 250 kbps, 1 Mbps or 2 Mbps data rates. (At lower data rates, the radio’s sensitivity is higher – on the order of -100 dBm at 250 Kbps.)

Alternatively, the Link Manager can configure and control a star network with up to eight clients, which transmit their data in a short burst after a programmable time code. It can also perform ‘temporary pairing’, which can be used to connect to multiple clients in a time-multiplexed fashion.

The Link Manager keeps a record of all timings in the network and handles the CRC checksum, acknowledgements and the retransmissions (up to three are allowed) by itself. It also controls all power modes such as power-down, idle, sleep and data mode, enabling the needed internal blocks only for the minimum required time, thus keeping average power consumption to a minimum.

For data transfers, the MCU only needs to access four separate 32-byte data buffers, two for RX and two for TX, via the 4-pin SPI interface.

Together, these features are sufficient for a range of applications such as wireless sensor networks, active RFID tags, body area networks, remote controlled toys, weather stations and wireless building controls (residential and industrial). The 1 mbps net data rate also allows the AS3940 to be used for high-quality audio streaming.

In addition, the Link Manager provides a high degree of interference immunity in all of these applications. Congestion in the 2.4 GHz band makes frequency changing and retransmissions unavoidable. Bluetooth uses Adaptive Frequency Hopping (AFH), which works by identifying fixed sources of interference and excluding them from the list of available channels spaced 1 MHz apart. The network then hops to an unoccupied channel in order to find spectrum free of interference. The downside of this technique is that it reduces the number of available channels.

Wi-Fi and ZigBee both rely on the Direct-Sequence Spread Spectrum (DSSS) technique, where the transmission remains centered on a wide channel. The transmitted data are multiplied by a pseudorandom sequence to create a wideband noise-like signal. In reception, the same pseudorandom sequence is applied to the signal to recover the original data. In the case of a narrowband interferer, de-spreading distributes the interference power over the bandwidth of the spreading code without degrading performance, but DSSS is more susceptible to jamming than other anti-interference techniques.

ams’s Link Manager adopts a different approach to the problem: Adaptive Channel Switching (ACS). Unlike the techniques just described, the Link Manager switches a channel only when data are not successfully received, even after three re-transmissions. The only task for the MCU is to initially write the same list of 16 channels for each device to ensure that the same channel is selected by both master and client. The frequently changing PRBS (pseudorandom binary sequence) number is unique for every client, thus creating a unique channel sequence and avoiding collisions.

The two possible failure modes for data communication are:
1) The master does not receive the data packet successfully and therefore does not reply with the ACK packet.
2) The data packet goes through, but the client does not receive the ACK packet.

In either case, both the master and the client select a new channel based on the PRBS number for the next communication slot. Channel switching is continued until either the communication is free from interference or the programmed number of lost communication slots is reached. As its name suggests, ACS adapts to the prevailing conditions the application is subject to, and thus provides a combination of both robustness and efficiency.

Of course, an RF IC requires more than just an effective protocol and channel hopping: the AS3940 also provides high RF performance, with excellent sensitivity of -100 dBm at 250 kbps. In a demonstration system, two AS3940 devices successfully transmitted data at 250 kbps over a 200 m range at 0 dBm output power. This low output power requirement supports operation in battery-powered applications.

How to quickly and easily implement a simple radio network
Suppliers of RF ICs appear to have become fixated on standards, which provide a rich feature set suitable for a wide range of applications, but add the cost of complexity and processing overhead. Put simply, RF protocols such as Bluetooth and ZigBee are expensive and difficult to implement.

The AS3940 transceiver IC shows that a different approach to wireless networking is possible. The Link Manager concept facilitates a very simple configuration and control by the MCU – with just 13 commands to master. This approach also offers the capability to implement a small star network or a point-to-point topology supporting data rates sufficient even for audio streaming.

For more information:
http://www.ams.com/AS3940

May 29, 2012

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