> > On 07/28/2017 06:57 AM, Kurt Van Dijck wrote: > > >So while _a_ transceiver may be spec'd to 1MBit during arbitration, > >CAN FD packets may IMHO exceed that speed during data phase. > > When the bitrate is limited to 1Mbit/s you are ONLY allowed to use 1Mbit/s > in the data section too (either with CAN or CAN FD). My point is that the requirements posed to a transceiver differ between arbitration & data phase for CAN FD. So while a transceiver does not know about CAN FD, it may allow higher bitrates for the data phase. > > >That was the whole point of CAN FD: exceed the limits required for > >correct arbitration on transceiver & wire. > > No. CAN FD is about a different frame format with up to 64 bytes AND the > possibility to increase the bitrate in the data section of the frame. > > >So I do not agree on the single bandwidth limitation. > > The transceiver provides a single maximum bandwidth. It's an ISO Layer 1 > device. > > >The word 'max-arbitration-bitrate' makes the difference very clear. > > I think you are mixing up ISO layer 1 and ISO layer 2. In order to provide higher data throughput without putting extra limits on transceiver & wire, the requirement for the round-trip delay to be within 1 bittime has been eliminated, but only for the data phase when arbitration is over. So layer 2 (CAN FD) has been adapted to circumvent the layer 1 (transceiver + wire) limitations. In fact, the round-trip delay requirement never actually did matter for plain CAN during data bits either. CAN FD just makes use of that, but is therefore incompatible on the wire. I forgot the precise wording, but this is the principle that Bosch explained on the CAN conference in Nurnberg several years ago, or at least this is how I remembered it :-) I haven't followed the developments of transceivers, but with the above principle in mind, it's obvious that any transceiver allows higher bitrates during the data segment because the TX-to-RX line delay must not scale with the bitrate. In reality, maybe not all transceivers will mention this in their datasheet. So whether you call it 'max-arbitration-bitrate' & 'max-data-bitrate' or 'max-bitrate' & 'max-data-bitrate' does not really matter (I prefer 1st) but you will one day need 2 bitrates. Kind regards, Kurt -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
