Multiplexed Networks for Embedded Systems: CAN, LIN, FlexRay, Safe-by-Wire...

6

Time-Triggered Protocols – FlexRay

6.1 Some General Remarks

By way of introduction to this chapter, I shall discuss a few matters which are common to all network architectures. You should know that this book is now about to change direction by assessing the properties of CAN (controller area network), describing its main limitations and devising new solutions to give new prospects for the decades ahead. First, CAN, created more than 20 years ago, is perfect for today's applications, but inevitably the passage of time reveals some of its limitations. In fact, as I will demonstrate shortly, CAN is very much event triggered (i.e. communications are initiated by events), and it lacks a ‘real-time’ orientation, or in other words, a ‘time-triggered’ philosophy. To overcome this, the first response was to create an upper layer called ‘TTCAN’, triggered by temporal events, to help revive CAN (see below).

The gross bit rate of CAN is limited to 1 Mbit s⁻¹ and future applications will be more orientated towards gross bit rates of 5–10 Mbit s⁻¹. So everything must be redesigned. The word ‘everything’ may surprise you, but it is true! Everything will have to be rethought and redesigned because the maximum CAN bit rate of 1 Mbit s⁻¹ practically corresponds to the limits of a technical philosophy in which we can still avoid too much mention of line propagation phenomena, reflection coefficients, stubs, Smith charts, etc., but above this rate we cannot consider designing physical layers and ...