FlexRay Active Star with Bit-Reshaper: enabling extended network topologies and more reliable communication

In order to operate as specified in the FlexRay standard, the sum of static and stochastic asymmetric delays must be lower than the maximum allowable lengthening or shortening of a bit at a receiving node (see Figure 1). Otherwise, the bit will not be decoded correctly and the frame and all its data will be discarded as erroneous. It can even lead to an inconsistent network state in which the frame is detected as valid by some nodes and as invalid by others.

How the Bit-Reshaper function operates

‘Bit-reshaping’ is a technique that reduces the extent of asymmetric delay across a FlexRay network. System designers can now implement bit-reshaping for the first time by using the AS8224 IC from austriamicrosystems.

Existing standard FlexRay Active Star devices already optimise the voltage level of incoming signals; this enables extended system configurations and decouples FlexRay lines at the Active Star. But the signal-processing mechanisms in the Bit-Reshaper additionally optimise the signal shape in the time domain, if an external clock input for the mechanism is available (see Figure 2).

Fig.2 : principal function of an Active Star Device with Bit-Reshaper. For higher resolution, click here.

The external clock input is converted to an internal sample clock using a configurable PLL that multiplies the input clock frequency by a factor of 2, 8 or 20. The sample clock is designed to oversample the incoming bit stream eight times, so for instance the sampling period (1 Tsample) is 12.5ns for a 10Mbps FlexRay network. The Bit-Reshaper decodes the incoming bit-stream and re-transmits a regenerated bit-stream with a total propagation delay of less than 238ns, which is within the maximum propagation delay of 250ns specified in the FlexRay electrical physical layer specification.

The total propagation delay includes the analogue delays at the receiver and transmitter stages, as well as the digital delay caused by the bit-reshaping logic, which requires 7 Tsamples (87.5ns) in total. As a result, overall system performance is not degraded and the data throughput of the FlexRay network is unchanged, but the asymmetric propagation delay is reduced to almost zero after passing through the FlexRay Active Star Device with Bit-Reshaper; the distortion in the incoming bit-stream is compensated completely at the digital level.

Measurement of the asymmetric delay of the outgoing bit-stream will show only small analogue effects – from the AS8224’s transmitter, and the clock jitter of the external clock input and the internal PLL – amounting to around 4ns. In principle, every bit regenerated by the Bit-Reshaper has a nominal length of 8 Tsamples. Due to small clock deviations between the transmitting and receiving nodes and the Bit-Reshaper, this ideal timing of 8 Tsamples per bit cannot be maintained for all bits, so the Bit-Reshaper has to insert single Tsamples at the right position in the FlexRay frame to synchronise to a ‘slow sender’ (that is, where the sender’s clock is slightly slower than the clock of the Bit-Reshaper). The sample is added at the Byte Start Sequence’s (BSS) high bit: in this way the impact on the receiver’s encoder is minimal, as synchronisation is performed at the BSS’s falling edge (the BSS’s high bit is the last bit before a new synchronisation).These lengthened bits are marked in blue in Figure 3.

Fig.3: incoming and outgoing bit-stream of the Bit-Reshaper. The incoming bit-stream comes from a slow sender. For higher resolution, click here.

In the case of a ‘fast sender’ (that is, where the sender’s clock is slightly faster than the clock of the Bit-Reshaper) the Bit-Reshaper does not shorten the corresponding bits in the outgoing bit-stream, since this would compromise any transceiver processing the modified bit-stream: the bits would be so short as to be close to the minimum allowable bit-time. Therefore the Bit-Reshaper uses a different strategy for processing the bit-stream of a fast sender.

A FIFO buffer is used to store the incoming bits temporarily before they are sent on (see Figure 4). Every bit retains the ideal length of 8 Tsamples (of the Bit-Reshaper’s clock). The reshaped bit-stream shows a slightly higher propagation delay than the incoming one, but it is no longer than it would be if the sender’s clock speed was the same as the Bit-Reshaper’s. An 8-bit FIFO buffer is sufficient to cover the worst case clock deviations (+/-1500ppm) in a FlexRay network.  

Fig.4 : Incoming and outgoing bit-stream of the Bit-Reshaper where the incoming bit-stream is from a fast sender. For higher resolution, click here.

A typical example of the signal waveform modification effected by the Bit-Reshaper is shown in Figure 5.

Fig. 5: results of the AS8224’s reshaping capabilities on an incoming signal with strong negative asymmetric delay (high bits are lengthened to 127.5ns). The outgoing bit-stream is perfectly reshaped to the nominal bit-length of 100ns.

• Channel 1: BP/Branch 4 output of AS8224 #2
• Channel 2: BM/Branch 4 output of AS8224 #2
• Channel 3: BP/Branch 1 input of AS8224 #1
• Channel 4: BM/Branch 1 input of AS8224 #1

Bit-Reshaper enables extended topologies

According to the FlexRay protocol, the total static and stochastic asymmetric delay must be within a budget of -37.5ns to +50ns across all communication paths between nodes within the entire FlexRay network. Delays in the physical layer are often particularly acute over long cable spans between nodes, or in links that pass electrically noisy areas in the vehicle.

This in turn restricts the network design choices available today to the system engineer, particularly in relation to:

• The number of Active Stars in the network

• The maximum cable length between any two nodes

• The maximum stub length for any node connected to a branch

• The number of ‘middle nodes’ (nodes without a bus termination) connected to a branch

• cable properties (impedance, damping, shielding)

• The level of electro-magnetic noise that can be tolerated

It is common for automotive designers to specify expensive shielded cables which minimise the asymmetric delays attributable to EMI, and which thus support an extended network topology.

Implementing Active Star devices with a Bit-Reshaper can eliminate the need for sheathed cable. Since the Bit-Reshaper regenerates the bit-stream, the entire asymmetric delay budget is available on every branch connected to the Active Star device, and the contributions to asymmetric delay of any incoming signal are reduced to zero before the signal is forwarded by the Bit-Reshaper to other branches.

The practical effect of the Bit-Reshaper technique is illustrated in Figure 6 – a signal transmission involving an Active Star device with and without a Bit-Reshaper. Guidance in the FlexRay standard’s Electrical Physical Layer Application Notes (EPLAN) indicates that a topology with one Active Star (one FlexRay Active Star can be made up of one or more Active Star devices) typically does not have enough margin to tolerate the effect of EMI. Through use of a Bit-Reshaper in the Active Star device(s), this margin can be significantly improved and larger topologies – even involving several cascaded Active Stars – are possible with standard, unsheathed cabling.

In the network without a Bit-Reshaper (see Figure 6), the allowable propagation delay of a single bit is exceeded by about 18ns. The same network topology with the Bit-Reshaper embedded in the Active Star has a margin of 12ns.

Fig. 6 part one. Asymmetric delay calculation in accordance with the FlexRay EPLAN For better resolution click here,

Fig. 6 part 2. For better resolution click here.

Summary 

The Bit-Reshaper inside the AS8224 is an innovation which significantly reduces existing limitations on the design of FlexRay networks. It enables the use of larger network topologies using two Active Star devices on the FlexRay bus: this is simply not possible without the Bit-Reshaper. It increases the network’s margin for tolerance of EMI, and can permit the use of cheaper, lighter unsheathed cable. The Bit-Reshaper function is implemented today in the AS8224 FlexRay transceiver from austriamicrosystems, which can be designed into existing Active Star products with the simple addition of a clock input signal to the AS8224. This can normally be provided by the host processor. 

About the authors:

Harald Gall  is Product Manager, Automotive Division at austriamicrosystems.

Monica Giardi  is Design Engineer at austriamicrosystems.

Christian Netzberger is Professor for Communications and Digital Circuit Design at the FH Joanneum, University of Applied Sciences in Kapfenberg, Austria.

Eric Schmidt is Senior Automotive Project Manager at TTTech Automotive GmbH