[Ovmsdev] DBC and Endian-ness

Mark Webb-Johnson mark at webb-johnson.net
Tue Feb 12 10:57:29 HKT 2019


Robert, Colin,

Thanks for your feedback. Very helpful, and I think I’ve got something workable now.

I found some more reference implementations (including Robert’s openpilot) and they all seem to use 0=big_endian and 1=little_endian. Either the specification is wrong, or everyone is ignoring it in the DBC’s they make.

I coded up something that seems to work (based on a few open source approaches). Haven’t tested it much, but will do more over the coming few days. I’ve committed what I’ve done so far, and it looks pretty good. Some notes on the (long list) of changes made in the past few days:

Support for both standard and extended message IDs added.

Re-worked everything to use the MyDBC memory-loaded DBC files. This includes retools, OvmsVehicleDBC, etc.

Made DBC a core module (no longer optional), as it is now extending tendrils into everything (can buses, metrics, etc).

Extended our core canbus class to be able to attach/detach a DBC at the can bus level.

Implemented the dbcSignal::Decode function (including a bunch of extensions to dbcNumber to make it more functional).

Extended our metrics system to support setting metric value via dbcNumber.

Extended retools to use canbus DBC’s and multiplexed signals for it’s unique message key determination. Also layed a foundational framework for ‘re dbc’ command tree (starting with ‘re dbc list’ to show RE messages and decodes dbc signals).

Provided a base implementation for OvmsVehicleDBC. This seems to work, but is largely untested (mainly because base functionality like dbc signal decoding is largely untested).

Now at the stage where this is starting to be usable. We should be able to create/load a DBC file, config it on a CAN bus, set vehicle type to DBC, and have metrics decoded from the CAN bus automatically. For the moment, we can use Wifi and Michael’s web interface file editor to maintain these DBC files.

I’m now mainly working on the RE stuff. Commands to directly edit DBC records (messages, signals, etc) on the box itself. Combined with the OvmsVehicleDBC vehicle, this should permit the basic functionality of reverse engineering CAN bus messages to produce metric values, without coding in C/C++.

It remains to be seen just how much can be done in this system. I think that by extending the DBC file format (can be with ‘#’ style comments, or other meta-data), we can even implement actions (such as lock/unlock), notifications (charge starting, etc), and other such high-level functionality.

Regards, Mark.

> On 11 Feb 2019, at 10:09 PM, Robert Cotran <robert at cotran.ca> wrote:
> 
> Hi Mark,
> 
> I don't know if you know about the openpilot project.  It's basically an open-source driving agent (autopilot). I've done a lot of work on it including developing the original port to making it work on Classic Tesla Model S with a friend of mine.
> 
> Part of what we needed to do was unpack and repack the Tesla DBC which has a mix of big and little-endian (Motorola and Intel) encoding.
> 
> Here is the github repo for the parser and packer (decoding/encoding) that we wrote in c++ to do it:
> 
> https://github.com/jeankalud/openpilot <https://github.com/jeankalud/openpilot>
> It's actually a branch from the real openpilot which now incorporates our packer/parser changes.
> 
> The secret sauce is here:
> 
> https://github.com/jeankalud/openpilot/tree/tesla/selfdrive/can <https://github.com/jeankalud/openpilot/tree/tesla/selfdrive/can>
> In parser.cc and packer.cc.  It should be relatively self-explanatory.
> 
> Let me know if you have any questions!
> 
> Rob
> 
> On 2019-02-10 9:55 PM, Mark Webb-Johnson wrote:
>> 
>> I’ve been going round and round in circles on this, most of the weekend, but not getting anywhere. I’ve rewritten the Decoder code four times so far, but just can’t get it to work. So calling for help here.
>> 
>> I attach the DBC specification. You can also find an extensive python library here:
>> 
>> https://github.com/eerimoq/cantools.git <https://github.com/eerimoq/cantools.git>
>> 
>> (for anyone playing with DBC, that seems an excellent library and set of command line tools)
>> 
>> The seemingly simple problem that I am trying to solve is:
>> 
>> Given a CAN frame, we need to decode a particular signal. We want the integer value (signed or unsigned) out of it.
>> 
>> The signal species the start bit, size (number of bits), endian-ness (big or little), and value type (signed or unsigned). The DBC specification says:
>> 
>> The start_bit value specifies the position of the signal within the data field of the frame. For signals with byte order Intel (little endian) the position of the least-significant bit is given. For signals with byte order Motorola (big endian) the position of the most significant bit is given. The bits are counted in a saw-tooth manner.
>> 
>> byte_order = '0' | '1' (* 0=little endian, 1=big endian *)
>> 
>> The byte_format is 0 if the signal's byte order is Intel (little endian) or 1 if the byte order is Motorola (big endian).
>> 
>> There are also factor and offset values for signals, but that is irrelevant for the moment (trivial to apply once we have the actual integer raw signal value extracted) and can be ignored for the moment.
>> 
>> Wikipedia explains endian-ness:
>> 
>> In big-endian format, whenever addressing memory or sending/storing words bytewise, the most significant byte—the byte containing the most significant bit—is stored first (has the lowest address) or sent first, then the following bytes are stored or sent in decreasing significance order, with the least significant byte—the one containing the least significant bit—stored last (having the highest address) or sent last.
>> 
>> Little-endian format reverses this order: the sequence addresses/sends/stores the least significant byte first (lowest address) and the most significant byte last (highest           address).
>> 
>> So, we have two strange things in the DBC specification: (1) the start bit is not always the first bit in the stream, and (2) this ’saw-tooth manner’ comment. So, I create a very simple DBC, and run it through the cantools dump to visualise it:
>> 
>> $ cat mark.dbc
>> 
>> BU_: WS200 MCN_Powertrain
>> 
>> BO_ 1 LittleEnd: 8 WS200
>>  SG_ OneByte : 7|8 at 0+ (1,0) [0|0] "" Vector__XXX
>>  SG_ TwoByte : 15|16 at 0+ (1,0) [0|0] "" Vector__XXX
>>  SG_ TwelveBit : 39|12 at 0+ (1,0) [0|0] "" Vector__XXX
>> 
>> $ cantools dump mark.dbc
>> ================================= Messages =================================
>> 
>>   ------------------------------------------------------------------------
>> 
>>   Name:       LittleEnd
>>   Id:         0x1
>>   Length:     8 bytes
>>   Cycle time: - ms
>>   Senders:    WS200
>>   Layout:
>> 
>>                           Bit
>> 
>>              7   6   5   4   3   2   1   0
>>            +---+---+---+---+---+---+---+---+
>>          0 |<-----------------------------x|
>>            +---+---+---+---+---+---+---+---+
>>                                          +-- OneByte
>>            +---+---+---+---+---+---+---+---+
>>          1 |<------------------------------|
>>            +---+---+---+---+---+---+---+---+
>>          2 |------------------------------x|
>>            +---+---+---+---+---+---+---+---+
>>      B                                   +-- TwoByte
>>      y     +---+---+---+---+---+---+---+---+
>>      t   3 |   |   |   |   |   |   |   |   |
>>      e     +---+---+---+---+---+---+---+---+
>>          4 |<------------------------------|
>>            +---+---+---+---+---+---+---+---+
>>          5 |--------------x|   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>                          +-- TwelveBit
>>            +---+---+---+---+---+---+---+---+
>>          6 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          7 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>> 
>>   Signal tree:
>> 
>>     -- {root}
>>        +-- OneByte
>>        +-- TwoByte
>>        +-- TwelveBit
>> 
>>   ------------------------------------------------------------------------
>> 
>> I also used 'cantools generate_c_source mark.dbc’ to generate a C decoder for this, and this is what it looks like:
>> 
>> int mark_little_end_unpack(
>>     struct mark_little_end_t *dst_p,
>>     const uint8_t *src_p,
>>     size_t size)
>> {
>>     if (size < 8u) {
>>         return (-EINVAL);
>>     }
>> 
>>     memset(dst_p, 0, sizeof(*dst_p));
>> 
>>     dst_p->one_byte |= unpack_right_shift_u8(src_p[0], 0u, 0xffu);
>>     dst_p->two_byte |= unpack_left_shift_u16(src_p[1], 8u, 0xffu);
>>     dst_p->two_byte |= unpack_right_shift_u16(src_p[2], 0u, 0xffu);
>>     dst_p->twelve_bit |= unpack_left_shift_u16(src_p[4], 4u, 0xffu);
>>     dst_p->twelve_bit |= unpack_right_shift_u16(src_p[5], 4u, 0xf0u);
>> 
>>     return (0);
>> }
>> 
>> Let’s look at that TwoByte value. This is little endian, so the least significant byte should come first in p[1] followed by the most significant in p[2]. So, why does the generator produce:
>> 
>>     dst_p->two_byte |= unpack_left_shift_u16(src_p[1], 8u, 0xffu);
>>     dst_p->two_byte |= unpack_right_shift_u16(src_p[2], 0u, 0xffu);
>> 
>> From what I can see, that means take p[1] and shift it left 8, then add on p[2]. That seems big endian, not little endian?
>> 
>> Let’s try a big endian example:
>> 
>> BO_ 3 BigEnd2: 8 WS200
>>  SG_ EightBit2 : 5|8 at 1+ (1,0) [0|0] "" Vector__XXX
>>  SG_ OneByte2 : 16|8 at 1+ (1,0) [0|0] "" Vector__XXX
>>  SG_ TwelveBit2 : 48|11 at 1+ (1,0) [0|0] "” Vector__XXX
>> 
>>                           Bit
>> 
>>              7   6   5   4   3   2   1   0
>>            +---+---+---+---+---+---+---+---+
>>          0 |----------x|   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          1 |   |   |   |<------------------|
>>            +---+---+---+---+---+---+---+---+
>>                          +-- EightBit2
>>            +---+---+---+---+---+---+---+---+
>>          2 |<-----------------------------x|
>>            +---+---+---+---+---+---+---+---+
>>      B       +-- OneByte2
>>      y     +---+---+---+---+---+---+---+---+
>>      t   3 |   |   |   |   |   |   |   |   |
>>      e     +---+---+---+---+---+---+---+---+
>>          4 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          5 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          6 |------------------------------x|
>>            +---+---+---+---+---+---+---+---+
>>          7 |   |   |   |   |   |<----------|
>>            +---+---+---+---+---+---+---+---+
>>                                  +-- TwelveBit2
>> 
>> Notice the weird alignment of EightBit2 and TwelveBit2? I presume that is the ’saw-tooth’ pattern talked about in the DBC specification.
>> 
>> A Tesla Model S encodes values like the battery temperature in CAN ID 0x102, and we decode this in C like this:
>> 
>> StandardMetrics.ms_v_bat_temp->SetValue((float)((((int)d[7]&0x07)<<8)+d[6])/10);
>> 
>> So that is little endian (d[7] is shifted right, followed by d[6] normally - so d[7] is the MSB and d[6] the LSB). But how to represent that in DBC?
>> 
>> BO_ 1 LittleEnd2: 8 WS200
>>  SG_ TwoByte : 47|11 at 0+ (1,0) [0|0] "” Vector__XXX
>> 
>>                           Bit
>> 
>>              7   6   5   4   3   2   1   0
>>            +---+---+---+---+---+---+---+---+
>>          0 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          1 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          2 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>      B   3 |   |   |   |   |   |   |   |   |
>>      y     +---+---+---+---+---+---+---+---+
>>      t   4 |   |   |   |   |   |   |   |   |
>>      e     +---+---+---+---+---+---+---+---+
>>          5 |<------------------------------|
>>            +---+---+---+---+---+---+---+---+
>>          6 |----------x|   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>                      +-- TwoByte
>>            +---+---+---+---+---+---+---+---+
>>          7 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+—+
>> 
>>     dst_p->two_byte |= unpack_left_shift_u16(src_p[5], 3u, 0xffu);
>>     dst_p->two_byte |= unpack_right_shift_u16(src_p[6], 5u, 0xe0u);
>> 
>> That doesn’t work. How about big endian?
>> 
>> BO_ 1 BigEnd2: 8 WS200
>>  SG_ TwoByte : 48|11 at 1+ (1,0) [0|0] "” Vector__XXX
>> 
>>                           Bit
>> 
>>              7   6   5   4   3   2   1   0
>>            +---+---+---+---+---+---+---+---+
>>          0 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          1 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          2 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>      B   3 |   |   |   |   |   |   |   |   |
>>      y     +---+---+---+---+---+---+---+---+
>>      t   4 |   |   |   |   |   |   |   |   |
>>      e     +---+---+---+---+---+---+---+---+
>>          5 |   |   |   |   |   |   |   |   |
>>            +---+---+---+---+---+---+---+---+
>>          6 |------------------------------x|
>>            +---+---+---+---+---+---+---+---+
>>          7 |   |   |   |   |   |<----------|
>>            +---+---+---+---+---+---+---+---+
>>                                  +— TwoByte
>> 
>>     dst_p->two_byte |= unpack_right_shift_u16(src_p[6], 0u, 0xffu);
>>     dst_p->two_byte |= unpack_left_shift_u16(src_p[7], 8u, 0x07u);
>> 
>> That seems correct.
>> 
>> Looking in the source for cantools dbc (cantools/cantools/database/can/formats/dbc.py), I see:
>> 
>> byte_order=(0 if signal.byte_order == 'big_endian' else 1),
>> 
>> This code also says the same thing (0 = big endian):
>> 
>> https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java <https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java>
>> 
>> boolean isBigEndian = "0".equals(splitted[2]);
>> 
>> WTF?
>> 
>> There is this (from googling), which makes things clear as mud:
>> 
>> https://github.com/ebroecker/canmatrix/wiki/signal-Byteorder <https://github.com/ebroecker/canmatrix/wiki/signal-Byteorder>
>> https://se.mathworks.com/help/vnt/ug/canpack.html <https://se.mathworks.com/help/vnt/ug/canpack.html>
>> https://github.com/ebroecker/SocketCandecodeSignals/blob/master/datenbasis.c#L112-L122 <https://github.com/ebroecker/SocketCandecodeSignals/blob/master/datenbasis.c#L112-L122>
>> https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java <https://github.com/julietkilo/CANBabel/blob/master/src/main/java/com/github/canbabel/canio/dbc/DbcReader.java>
>> 
>> My conclusions are that:
>> 
>> The DBC specification is incorrect, in that 0 is big endian, and 1 is little endian in the public code we see.
>> 
>> and/or
>> 
>> The DBC specification is incorrect, in that all CAN bus byte ordering is little endian, and the byte_order setting only affects the interpretation of the bit_ordering, and saw-tooth decode/encode.
>> 
>> But that really makes no sense to me. This is supposed to be the defacto standard for automotive CAN bus signal definition - how can it be such a mess?
>> 
>> Help?
>> 
>> Mark
>> 
>> 
>> 
>> 
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