CAN FD system design

icc 215 CAN FD system design Dr. - Ing. M. Schreiner Daimer Research and Deveopment Abstract The objective of this paper is to give genera design rues for the physica ayer of CAN FD networks. As an introduction infuencing parameters are anayzed and physica reationships are shown. Critica vaues of typica components are given. The main section wi then present a systematica anaysis of basic CAN FD topoogies (e.g. star, bus or sub topoogy). The topoogies wi be described by geometrica parameters and the respective physica characteristics wi be derived. Finay an assessment of the possibe baud rates of given network topoogies as a function of the geometrica parameters wi be provided. Introduction After its introduction in 212 [1] CAN FD quicky turned out to be the next big thing for in vehice networking in addition to the introduction of automotive Ethernet. Meanwhie CAN FD has become a new ISO standard and many automakers are about to integrate CAN FD into the next generation of their vehices [4], [5]. One of the main benefits of the new CAN FD protoco is its abiity to transmit the data phase of the frame with higher speed. Under ab conditions even 1 Mbits/s and more have been demonstrated for sma networks. The abiity of CAN or CAN FD to run on neary any kind of network topoogy is its strength and weakness at the same time. Many CAN users have gained experience in designing Cassica CAN networks at 125 kbit/s 5 kbit/s but up to now may do not have a feeing for the appropriate baud rate of a CAN FD topoogy. The focus of the paper is to give an overview about the characteristics of frequenty used CAN topoogies in terms of CAN FD data phase speed. The resuts given in this paper enabe CAN users to determine the maximum baud rate of frequenty used topoogies for save operation under mass production. Design rues for Cassica CAN Cassica CAN is aso a basic part of CAN FD (bus access etc.) and hence when designing a CAN FD network the rues for proper arbitration, acknowedge etc. must not be negected. Principay this is out of the scope of this paper; the basic reationships are given in [6] and [7]. When designing smaer CAN FD networks targeting at higher baud rates (e.g. 2 Mbit/s) the rues for the data phase are the imiting factor in most cases, not the rues for Cassica CAN. However in arge networks (e.g. industria appications) the imiting factor might be arbitration and not the CAN FD data phase. Design rues for CAN FD The principas of the robustness of the CAN FD protoco concerning bit timing, cock toerance and any kind of asymmetry of the received CAN signas have aready been given in [2] and [3]. However these are theoretica vaues based on ogic signas. Now the existing gap to rea CAN FD topoogies and practica impementations wi be bridged! The crucia point for Cassica CAN systems is the interaction between transmitter and receiver (i.e. the deay of the signas), whereas the main imitation in CAN FD systems is the asymmetry of the received signas. The toerabe shrinking and growing of bits has been expressed in [3] as phase margin. Since dominant and recessive bits can either shrink or grow with different consequences for the protoco two vaues (PM1 and PM2) have been defined. These are depending on the chosen bit time settings shown in figure 1 for both vaues and different sampe point positions in the range between 5 kbit/s and 2 Mbit/s. If the asymmetry of the received bits exceeds the vaues for PM1 or PM2 communication wi break down. 7-1

icc 215 14 12 1 8 6 8 signa at node view of node: when quant. error is min. when quant. error is max. bit 1 bit 5 bit 6 vaues defined in DIS/ISO 11898-2:215. A transceiver compiant to the ISO shoud not be worse. This part is marked ight bue in figure 2. phase margin (ns) 8 6 4 2 6 8 76 8 76,4,6,8 1, 1,2 1,4 1,6 1,8 2, Baudrate (Mbit/s) Figure 1: Phase margins for CAN FD 6 The phase margin in a rea CAN system is the maximum aowabe asymmetry before communication breaks down. To be sure that a CAN FD system wi work propery under any circumstance a parts in a system that contribute to bit asymmetry have to be known incuding toerances and varying operation conditions (e.g. temperature and aging). A these parts together must never exceed the phase margin. This is iustrated in figure 2. breakdown of communication 6 6 PM1 PM2 6 6 nomina bit time phase margin 2 sync_seg bit boundary sampe point Figure 2: Distribution of phase margins 6 6 The aowed range of deviation from the nomina bit time of a singe bit are the bue parts in figure 2, the red shaded area are the imits given by PM1 and PM2. The first part which is inherent in any CAN or CAN FD system is the characteristics of the transceiver chips. Each transceiver has a typica deay and a typica asymmetry. Since transceiver timing characteristics spread depending on batch, temperature and aging it is recom-mended to use the imiting 56 56 phase margin 78,5 57 57 78,5 6 phase margin 1 phase_seg1 phase_seg2 sampe point (%) breakdown of communication next bit intrinsic asymmetry of transceiver (cf. ISO) osciator jitter, EMC jitter, PCB etc. aowabe asymmerty for topoogy ony time 6 6 The second part accounts for a other kind of asymmetries that are part of the physica ayer not incuding the transceiver and the topoogy itsef. It is subdivided into many smaer parts and it depends on the specific use case of the CAN FD system what to incude and what vaues to take into consideration. In the foowing major vaues for typica automotive appications are given: Time interva error (TIE): the cock of CAN FD controers is usuay derived from PLL circuits incuded in the µc. Any PLL is affected by phase noise causing a jitter between consecutive bits. This short term jitter is equivaent to bit asymmetry and it adds up to the overa asymmetry by a factor of four because a CAN bit is defined by two sopes at the transmitter side which have to be samped at the receiver side (i.e. worst case: the first sope is deayed by transmitter and samper at the receiver and vice versa for the second sope). Typica vaues are 5 ns... 2 ns. Pease note that the TIE is not equivaent to the static osciator deviation which is aready incuded in the cacuation in [3] and which is mainy defined by the crysta being used. EMI jitter: If a CAN or CAN FD system is exposed to EM radiation or fast common mode transients on the bus ines the signas provided by the receivers wi be affected by jitter. Again this jitter wi be seen by a controer as asymmetry of the received signa. It is difficut to define a distinct vaue for this, but EMI measurements have shown, that 5 ns is a reasonabe vaue to account for this. However depending on the depoyment of the system this might be ess or even more. Logic asymmetry: the signas exchanged between the state machine in the CAN FD controer and the transceiver (, ) are aso affected by asymmetry resuting from the input/output switching pads of the siicon chips and the PCB capacitance. The typica vaue for this is 1 ns for the transmitting and receiving node. 7-2

icc 215 Finay the third part which is iustrated dark bue in figure 2 is the asymmetry caused by the topoogy, not incuding the transceivers. The assessment of this part wi be treated in detai in the foowing chapters. Eventuay it is recommended to account for future extensions of a network which means not to go directy to the imits of the phase margin when defining a new CAN FD system. Assessment of CAN FD topoogies In order to estimate the asymmetry of a specific topoogy it is necessary to measure a communication reationships between a nodes incuded. This means to measure n² signas if n is the number of nodes in the network. Since any CAN transmitter is simutaneousy a receiver of his own signa, the so caed oop back signas have to be considered as we. It is recommended to use varying test patterns in the message to find the worst case bit combination. In the end there wi be one communication reationship with the highest asymmetry defining the worst case of the specific topoogy. This vaue has to be used for the cacuation given above. The principe of the measurement approach that has been used for a topoogy measurements is shown in figure 3. Basicay the approach can be appied to simuation technoogy as we as to measurement technoogy. In tota approximatey different topoogy variations have been tested physicay and anayzed automaticay by software. In the end one point in the graphs of the next chapters represents the worst case asymmetry of an entire topoogy, i.e. one graph shows a set of characteristic curves consisting of many different topoogies. The basic conditions of a measurements were: test message 2 Mbit/s, transceiver NXP TJA143T, room temperature, suppy votage 5 V ± 5%, choke 51 µh bifiar, cabe: FLRY 2 x,35 mm² (PVC standard CAN cabe). The assessment of the topoogies is divided into four parts: Assessment based on the signa, spit assessment of oop-back signa and signas from a other nodes. Assessment based on the bus signa at the respective node based at 5 mv (DR) and 9 mv (RD) threshod, spit assessment of oop-back signa and signas from a other nodes. In the foowing this wi be referred to as virtua signa. An exampe for this is shown in figure 4 where the asymmetry of the received signa (bue ines) is determined twice, based on the signa (upper graph) and on the differentia bus signa (ower graph). SE scope Tx1 (trigger) trigger once at a nodes scope bus 1 scope bus 2 scope 1 scope 2 SE µc SE CAN_H CAN_L diff CAN topoogy under test diff CAN_H CAN_L SE µc measure once at a nodes CAN node 1 for a trigger positions CAN node 1 µc CAN_H CAN_L resut: matrix with n² measurements CAN_H CAN_L µc CAN node n CAN node n-1 Figure 3: measurement approach for systematic topoogy assessment 7-3

icc 215 The magenta circes depict the switching points of the virtua signa. This dua assessment is necessary since good transceivers are fitering out ringing on the bus fairy good with their hysteresis behavior. In many situations an improve-ment of the asymmetry on the pin can be observed paradoxicay with increasing ringing to some extent. This can be deceiving for two reasons: Firsty the fitering of the ringing by the transceiver s hysteresis shows a distinct fa of the ciff behavior resuting in a jumping up asymmetry when reaching the imits of the hysteresis fitering. Secondy the fitering behavior is not specified in the DIS/ISO 11898-2:215 which means that two unequa CAN transceiver impemen-tations might behave differenty whie receiving CAN signas affected by ringing. Thus an assessment of a CAN topoogy shoud be based primariy on the bus signas and additionay on the signa deivered by a common CAN transceiver in order to guarantee stabe and reproducibe resuts. Finay it is up to the system designer to decide whether to trust in the signa or to consider the virtua signa based on threshods of the differentia bus signa. Finay it coud be distinguished between asymmetry af-fecting PM1 and PM2 when evauating the resuts. If one of both dominates the appropriate setting of the samping points coud achieve an optimization of the system s reserves. However in this paper ony the worst case is given not distinguishing between PM1 and PM2 in order not to dissipate one s energies in detais, otherwise the number of graphs woud have to be doubed. It has to be pointed out, that the given topoogy measurements incude the intrinsic asymmetry of an NXP TJA143 transceiver at room temperature (approx. 2 C). If the worst case defined by the ISO sha be taken into account (which is recommended to consider temperature and aging effects), then the transceivers typica vaues given in the datasheet by NXP have to be subtracted from the measured asymmetry vaues in a first step and in a second step the worst case asymmetry vaues given in the DIS/ISO 11898-2:215 have to be added. As a first approximation this represents the worst case asymmetry that coud be expected from a topoogy. Pease note that cabes might change their parameters over temperature which woud not be incuded in this approximation. / signa (V) differentia bus signa (V) 4 3 2 1 3 2 1-1 ogic transmitted ogic received ogic bit size dominant bit size based on rea transceiver () 992n 57,2n 491,2n 58,8n 491,2n differentia bus signa at receiver.5v/.9v transceiver threshods virtua bit size based on diff. bus virtua dominant bit based on differentia bus signa 9mV t dom. 916,8n 5mV 582,4n 417,6n 582,4n 417,6n 5 t bus dom. recessive bit size based on rea transceiver () t rez. virtua recessive bit based on differentia bus signa ringing diff. bus signa t bus rez. ringing has to decay beow 5mV, after that virtua togges virtua switching threshods DR 5mV / RD 9mV 71,5µ 71,µ 72,µ 72,25µ 72,5µ 72,µ 73,µ 73,25µ 73,5µ 73,µ Figure 4: Definition of asymmetry based on rea signa and virtua signa t(s) 7-4

icc 215 Point to point topoogy This is the simpest topoogy in which two variations are possibe: 1st terminated on both ends or 2nd terminated at one end. Termination at one end is sometimes beneficia from a user point of view but not from a signa integrity point of view, as can be seen from the graphs in fig. 5. The measurement stops at approx. 3 m because the communication broke down at this ength at 2 Mbit/s. Especiay the oop back signa (i.e. the signa that is received by a transmitting node itsef) is affected by strong ringing and therefore shows jumping up asymmetry with increasing ine ength. If the point to point topoogy is terminated at both sides the asymmetry of the oop back signa remains on a ow eve basicay determined by the intrinsic asymmetry of the transceiver. The received signa at the other node shows increasing asymmetry with increasing ine ength mainy coming from the dispersion of the transmission ine smoothing the sopes of the differentia CAN signa. Anyhow ong inks are possibe with a point to point topoogy. As standard PVC cabe was used for this measurement the asymmetry coud be improved a ot if a cabe with ower dispersion woud be used, e.g. FLR9Y (PP) or FLR2X (PE) instead of FLRY (PVC). The assessment based on the and the virtua signas (i.e. the differentia bus signa) are pretty simiar in this case. As can be seen equay terminated point to point inks are benchmark with regard to bit asymmetry. onesided termination = (1, 2, 4, 8, 16, 32) m both-sided termination = (1, 2, 4, 8, 16, 32, 64, 128) m E1 E2 E1 E2 6 12 E1/2 = end node = bus ength = terminated node 35n 3n 25n 2n 15n evauation based on rea signa both-sided term. onesided term. 35n 3n 25n 2n 15n evauation based on virtua signa both-sided term. onesided term. 1n 1n 5n 5n 2n communication AB 2n communication AB 16n 12n 8n both-sided term. onesided term. 16n 12n 8n both-sided term. onesided term. 4n communication stopped at 32m with test baudrate due to high oop back asymmetry 4n communication stopped at 32m with test baudrate due to high oop back asymmetry 2 4 6 8 1 12 transmission ine ength (m) Figure 5: Asymmetry of point to point ink 2 4 6 8 1 12 transmission ine ength (m) 7-5

icc 215 Line topoogy The idea ine topoogy is frequenty used in FexRay systems. Basicay it is a point to point ink that is expanded with in-between notes stricty avoiding stubs. This means that a nodes that are ooped into the bus ines need 4 pins instead of 2. This is aso the reason why this topoogy is quite unpopuar. This topoogy has the advantage that it reduces refections to a minimum. Sometimes this is aso referred to as daisy chain topoogy whereas this term is not appropriate in this context. This topoogy has been investigated in different configurations: 1 st equay spaced nodes, 2 nd custering of nodes at one end, 3 rd custering of nodes in the midde, 4 th having mutipe custers and 5th random ine distribution. In principe a five configurations show simiar behavior for which reason ony graphs for 1 st and 2 nd configuration are given. ine topoogy equay spaced n =.. 14 mid nodes a = 1, 2, 4, 8, (16), (32), (64) m E 1 T 1 T 2 T n E 2 12 12 E 1 a a a a ine topoogy accumuation of nodes cose to one end node n =.. 14 mid nodes a = 1, 2, 4, 8m b = a + (1, 2, 4, 8) m partiay: (16), (32), (64) T 1 T 2 T n E 2 12 12 a a a b E = end node T = mid node a/b = ine segment ength = terminated evauation based on rea signa evauation based on virtua signa 16n 12n 8n 4n asymmetry increasing with number of nodes 6 nodes 7 nodes 8 nodes 1 1 16 nodes 16n 12n 8n 4n 6 nodes 7 nodes 8 nodes 1 1 2n 2n communication between nodes 16n 12n 8n 4n asymmetry increasing with number of nodes asymmetry increasing with bus ength 6 nodes 7 nodes 8 nodes 1 1 16 nodes 16n 12n 8n 4n 6 nodes 7 nodes 8 nodes 1 1 2 4 6 8 1 12 2 4 6 8 1 12 tota bus ength (m) tota bus ength (m) Figure 6: Asymmetry of ine topoogy with equay spaced nodes 7-6

icc 215 If the communication is evauated based on the signa (fig. 6 on the eft) it can be seen, that especiay the oop back signa shows increasing asymmetry with increasing number of nodes at smaer transmission ine engths. This is due to fast ringing that occurs at the oop back signa of the in-between nodes and it becomes ess if the bus ine gets onger. The asymmetry of signas received from other nodes shows a simiar behavior but at onger transmission ine engths the asymmetry rises continuousy which is due to dispersion of the used transmission ine. Again this effect coud be improved if e.g. FLR9Y (PP) or FLR2X (PE) woud be used instead of FLRY (PVC). It can be observed, that a asymmetry vaues in figure 6 and foowing are a in a certain range. This is visuaized by the coored area around the curves. A variations that have been tested ie within this area. It is very ikey that variations of the topoogy that do not exceed the range of the tested variabes (e.g. number of nodes, maximum ine engths) aso ie within these areas, however this has not been tested. The coored areas shoud not be interpreted as strict boundary vaues; they shoud give an overview and orientation to CAN FD system designers. If the evauation is based on the virtua signa quite a simiar behavior can be observed, however the spread is arger. If nodes are custered at one end of the bus having a distant end termination node the behavior is again very simiar (fig. 7). The distance of the nodes within the custer have neary no infuence on the asymmetry, though with rising distance of the end node the overa bus ength grows which goes aong with dispersion effects of the transmission ine and increasing asymmetry. In comparison to the topoogies presented in the foowing chapters the ine topoogy shows very itte asymmetry. If it is buit up in an appropriate manner stricty avoiding stubs and ooping through the CAN signa through ECUs by means of 4 pins it has asymmetry vaues on the eve of a point to point communication ink. For CAN FD systems targeting at high communication speed in the data phase this kind of topoogy is the most suitabe. evauation based on rea signa evauation based on virtua signa 16n 12n 8n a=1m every custer: b=a+(1,2,4,8)m a=2m 9 nodes additionay b=a+(...16,32,64)m 16n 12n 8n same principe for a / b as on eft graph 9 nodes 4n 2n 16n 12n 8n 4n a=4m 9 nodes a=1m a=2m every custer: b=a+(1,2,4,8)m a=4m a=8m additionay b=a+(...16,32,64)m a=8m 2 4 6 8 1 12 tota bus ength (m) 4n 2n 16n 12n 8n 4n 9 nodes same principe for a / b as on eft graph 2 4 6 8 1 12 tota bus ength (m) Figure 7: Asymmetry of ine topoogy with custered nodes 7-7

icc 215 Bus topoogy with stubs This may be the most popuar and most frequenty used CAN topoogy. The big question is how ong the stubs can be since they are a source of refections. The stub topoogy was investigated with 1 st stubs of equa ength and equay spaced, 2 nd same as 1 st configuration but one of the stubs was offset and 3 rd configuration were randomy distributed stubs of varying ength. The ast one is not incuded in the paper because the resuts were comparabe to the first two configurations. cassic bus topogy with stubs stubs with equa ength, equay spaced E 1 E 1 12 a a s a a a T 1 T 2 cassic bus topogy with stubs one offset stub 12 s T 1 T 2 s a a a s...... offset stub T n T n o s 12 n =.. 6 stubs a = (1, 2, 4, 8) m E 2 a = (1, 2, 4, 8) m s = (.25,.5, 1, 2, 4) m 12 n = 2.. 4 stubs ast stub is offset E 2 s = (.25,.5, 1, 2, 4) m o = s + (.25,.5, 1, 2, 4) m E = end node T = mid node a/b = ine segment ength = terminated 6n 5n 4n 3n 2n 1n 6n 5n 4n 3n 2n 1n evauation based on rea signa asymmetry increasing with number of stubs and stub ength asymmetry increasing with number of stubs and stub ength 1 stub 2 stubs 3 stubs 4 stubs 5 stubs 6 stubs s =,25 m s =,5 m s = 1 m s = 2 m s = 4 m 1 stub 2 stubs 3 stubs 4 stubs 5 stubs 6 stubs s =,25 m s =,5 m s = 1 m s = 2 m s = 4 m 2 4 6 main bus ength (m) 6n 5n 4n 3n 2n 1n 6n 5n 4n 3n 2n 1n evauation based on virtua signa 1 stub 2 stubs 3 stubs 4 stubs 5 stubs 6 stubs 1 stub 2 stubs 3 stubs 4 stubs 5 stubs 6 stubs s = 2m s = 1m s =,5m s =,25m s = 2m s = 1m s =,5m s =,25m 2 4 6 main bus ength (m) Figure 8: Asymmetry of bus topoogy with stubs of equa ength, equay spaced 7-8

icc 215 If the resuts are assessed based on the signa (fig.8 eft) a pattern comparabe to the ine topoogy can be observed: an increasing asymmetry with an increasing number of nodes at smaer bus engths. For arger bus engths (in this case the tota bus ength was imited to a itte ess than 6 m) the observed asymmetry seems to be surprisingy ow, even if the stubs become quite ong (e.g. 4 m). However if the assessment is based on the virtua signa (i.e. a bad receiver is assumed) competey contradictory properties of the topoogy regarding asymmetry can be seen (fig. 8 right). Especiay with increasing stub ength the asymmetry bounces up for both, oop back signa and. The dependency on the stub ength is visuaized in figure 8 right side by different shadings of the coored area. An anaysis of the differentia bus signas shows that the stubs cause refections that affect oop back signas as we as communication signas between nodes. The ringing frequency is dependent on the sub ength and drops sower under the receiver threshod with increasing stub ength. This kind of topoogy is a good exampe for the capabiity of modern CAN transceivers to fiter out ringing. However a system designer can ony rey on that if the imits of the transceivers fitering capa-biities are we known. If this is not the case it is rather recommended to consider the virtua signa instead of the signa for system design. Finay figure 9 shows the resuts if ony one stub is engthened and the other stubs maintain a stub ength of 1 m. The tota asymmetry is ess in this case however the principe reationships stay the same. More tests with randomy varying stub engths and distance between the stubs aso show simiar behavior. A particuary bad situation has been found when mutipe stubs are connected to the bus ine at the same position. Cassic bus with stubs topoogies are popuar among many CAN system designers, however the given resuts show that this is not the optimum topoogy for CAN FD systems targeting at high communication speeds. If it shoud be used anyway for CAN FD keep the stubs as short as possibe. 7n 1 sub / 1 offset stub 2 subs / 1 offset stub 6n 3 subs / 1 offset stub 5n 4n 3n 2n 1n 7n 6n 5n 4n 3n 2n 1n evauation based on rea signa other stubs: s = const. = 1m other stubs: s = const. = 1m offset stub ength o = 1,25 m o = 1,5 m o = 2 m o = 3 m o = 5 m 1 sub / 1 offset stub 2 subs / 1 offset stub 3 subs / 1 offset stub offset stub ength o = 1,25 m o = 1,5 m o = 2 m o = 3 m o = 5 m 7n 6n 5n 4n 3n 2n 1n 7n 6n 5n 4n 3n 2n 1n evauation based on virtua signa offset stub ength o = 1,25 m o = 1,5 m o = 2 m o = 3 m o = 5 m s = const. = 1m 1 sub / 1 offset stub 2 subs / 1 offset stub 3 subs / 1 offset stub s = const. = 1m 1 sub / 1 offset stub 2 subs / 1 offset stub 3 subs / 1 offset stub offset stub ength o = 1,25 m o = 1,5 m o = 2 m o = 3 m o = 5 m 2 4 2 4 main bus ength (m) main bus ength (m) Figure 9: Asymmetry of bus topoogy with 1m stubs and one offset stub 7-9

icc 215 Star topoogy with ferrites The star topoogy used to be patented by Daimer-Benz in the 199s (DE4235616) and thus it is mainy depoyed in Daimer trucks, busses and passenger cars. The physica principe is shown in figure 1. Figure 1: star topoogy principe The main advantage of this configuration is that CAN nodes can be added or removed to a topoogy in a simpe manner not disturbing the principa structure of the network. Since the patent is expired by now some other car makers use this kind of topoogy as we. The key point of this topoogy is the usage of ferrites depoyed in the star center. Star topoogies not using ferrites in the star center are ony suitabe for very ow baud rates and thus they are not considered in this paper. Different configurations of this topoogy have been tested. In the 1 st configuration a branches of the star are varied with equa ength, 2 nd haf of the branches are extended in ength and 3 rd branches with equa ength and one offset branch with extended ength were tested. Since the ast one shows simiar resuts compared to the 2 nd one, these graphs wi be omitted. First the assessment based on the signa is regarded. In figure 12 eft the oop back asymmetry seems to be fairy ow whereas the is affected by considerabe asymmetry, which is rising with branch ength and with the number of branches. Obviousy the worst case is a arge number of branches and onger branch engths, however with few branches and moderate branch ength ow asymmetries sti can be achieved. A simiar resut can be seen in figure 13 where short and ong branches are combined. Pease note that the maximum branch engths in this case are onger than those in figure 12 which expains the overa higher asymmetry. A star topoogy wi aways be affected by refections as we as stub topoogies. That becomes evident when ooking at the virtua signa that accounts for ringing on the bus. Especiay for a higher number of nodes the asymmetry bounces up even at moderate branch engths. In figure 12 many data points are missing for this case because an asymmetry vaue coud not be defined anymore since the ringing persists for the whoe bit time of the baud rate that was used for the measurements (2 Mbit/s). Again this is a good exampe for the capabiity of modern transceivers to fiter out ringing. star topoogy incuding star with integrated ferrites equa ength / mixed enth T 1 T 4 a branches equa ength: n = 3.. 12 branches = (1, 2, 4, 8, 16) m T 2 6 T n-1 branches with mixed ength: T 3 T n n = 4, 8, 12 branches even = (1, 2, 4, 8, 16) m odd = even+ (1, 2, 4, 8, 16) m T = node = ine segment ength = terminated star with 6 Figure 11: Star topoogy tested configurations 7-1

icc 215 Anyhow there are configurations of the star topoogy (e.g. branches 2 m and no more than 8 nodes) where higher baud rates are possibe, even if the virtua signa is used for estimation. However for genera CAN FD system impementations the star topoogy is ony appicabe for baud rates which are in the range of Cassica CAN. Concusion An extensive measurement series of different CAN FD topoogy structures with a ot of varying parameters has been performed. In the process, the CAN FD signa asymmetries have been anayzed based on the as we as on a virtua signa based on the differentia bus signa. Athough the number of assessed variations was huge (approximatey in tota) of course they cannot cover a kinds of CAN FD topoogies that might occur in the fied. Nevertheess the given measure-ment resuts can give a good basic overview about the typica behavior of particuar topoogies and they might be a good hep for a CAN FD system designer to configure CAN FD networks in an appropriate manner. If shoud be noted that the presented resuts are vaid ony at room temperature. The incusion of temperature dependent behavior woud have doubed the number of graphs and woud have gone far beyond the scope of this paper. Large changes over temperature have to be expected if PVC cabe is used, otherwise the main infuence comes from the transceivers which can be accounted for by appying the worst case ISO vaues. If the CAN FD system design is targeting at high baud rates in the data phase (e.g. 2 Mbit/s or above) it is evident, that the best resuts can be achieved with the point to point and with the ine topoogy. Especiay for conservative system designers that do not want to toerate the uncertainty of ringing in the network the pure ine topoogy is the ony safe choice. Anyhow the popuar bus with stubs topoogy can be used for CAN FD, even at higher baud rates but in this case it is recommended to keep the stubs as short as possibe and rather increase the overa bus ength than aowing for onger stubs. However the system designer has to ive with the presence of ringing on the bus ines which has to be controed carefuy. Eventuay the ferrite star topoogy can hande fast CAN FD signas but ony with ow branch engths and a moderate number of branches. After a this kind of topoogy might be hepfu to fexiby interconnect CAN FD devices that are cose to each other, e.g. mutipe ECUs in an eectric contro cabinet. If baud rate doesn t matter the resuts show that the system designer has much more freedom to choose between different topoogy structures. The given graphs can be a source of orientation. Dr.-Ing. Marc Schreiner Daimer AG Research and Deveopment Wihem-Runge-Straße 11 DE-8981 Um References [1] CAN with Fexibe Data-Rate - Forian Hartwich,, icc 212, Neustadt an der Weinstraße [2] Bit time requirements for CAN FD - Forian Hartwich,, icc 213 Paris [3] Robustness of a CAN FD Bus System About Osciator Toerance and Edge Deviations A. Mutter, icc 213 Paris [4] Safeguarding CAN FD for appications in trucks - M. Schreiner, H. Leier, M. Zerzawy, T. Dunke and J. Dorner, CAN newsetter 3/213 [5] CAN FD from an OEM point of view, M. Schreiner, H. Mahmoud, M. Huber S. Koç, J. Wadmann,, icc 213 Paris and CAN Newsetter 2/214 [6] Berechnung des Bit Timings bei CAN Bus Systemen / Tei1 und Tei2 Kaus Dietmayer, Eektronik 21/1997 und Eektronik 22/1997 [7] The Configuration of the CAN Bit Timing Forian Hartwich, 6th Internationa CAN Conference 2nd to 4th November, Turin (Itay) 1999 7-11