Messages in this thread | | | Subject | Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN fixed transceiver bindings | From | Oliver Hartkopp <> | Date | Fri, 28 Jul 2017 20:33:28 +0200 |
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Hi Kurt,
On 07/28/2017 03:02 PM, Kurt Van Dijck wrote:
>>> 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 just checked an example for a CAN FD qualified transceiver
http://www.nxp.com/products/automotive-products/energy-power-management/can-transceivers/high-speed-can-transceiver-with-standby-mode:TJA1044
where it states:
The TJA1044T is specified for data rates up to 1 Mbit/s. Pending the release of ISO11898-2:2016 including CAN FD and SAE-J2284-4/5, additional timing parameters defining loop delay symmetry are specified for the TJA1044GT and TJA1044GTK. This implementation enables reliable communication in the CAN FD fast phase at data rates up to 5 Mbit/s.
and
TJA1044GT/TJA1044GTK
- Timing guaranteed for data rates up to 5 Mbit/s - Improved TXD to RXD propagation delay of 210 ns
> 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.
The question to me is whether it is right option to specify two bitrates OR to specify one maximum bitrate and provide a property that a CAN FD capable propagation delay is available.
E.g.
max-bitrate max-data-bitrate
or
max-bitrate canfd-capable // CAN FD capable propagation delay available
I assume the optimized propagation delay is 'always on' as the transceiver is not able to detect which kind of bits it is processing. That's why I think providing two bitrates leads to a wrong view on the transceiver.
Regards, Oliver
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