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Max Staudt says: ==================== This is the can327 driver. It does a surprisingly good job at turning ELM327 based OBD-II interfaces into cheap CAN interfaces for simple homebrew projects. Please see the included documentation for details and limitations: Documentation/networking/device_drivers/can/can327.rst ==================== Link: https://lore.kernel.org/all/20220618195031.10975-1-max@enpas.org/ Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
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.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-3-Clause) | ||
can327: ELM327 driver for Linux SocketCAN | ||
========================================== | ||
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Authors | ||
-------- | ||
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Max Staudt <max@enpas.org> | ||
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Motivation | ||
----------- | ||
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This driver aims to lower the initial cost for hackers interested in | ||
working with CAN buses. | ||
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CAN adapters are expensive, few, and far between. | ||
ELM327 interfaces are cheap and plentiful. | ||
Let's use ELM327s as CAN adapters. | ||
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Introduction | ||
------------- | ||
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This driver is an effort to turn abundant ELM327 based OBD interfaces | ||
into full fledged (as far as possible) CAN interfaces. | ||
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Since the ELM327 was never meant to be a stand alone CAN controller, | ||
the driver has to switch between its modes as quickly as possible in | ||
order to fake full-duplex operation. | ||
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As such, can327 is a best effort driver. However, this is more than | ||
enough to implement simple request-response protocols (such as OBD II), | ||
and to monitor broadcast messages on a bus (such as in a vehicle). | ||
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Most ELM327s come as nondescript serial devices, attached via USB or | ||
Bluetooth. The driver cannot recognize them by itself, and as such it | ||
is up to the user to attach it in form of a TTY line discipline | ||
(similar to PPP, SLIP, slcan, ...). | ||
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This driver is meant for ELM327 versions 1.4b and up, see below for | ||
known limitations in older controllers and clones. | ||
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Data sheet | ||
----------- | ||
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The official data sheets can be found at ELM electronics' home page: | ||
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https://www.elmelectronics.com/ | ||
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How to attach the line discipline | ||
---------------------------------- | ||
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Every ELM327 chip is factory programmed to operate at a serial setting | ||
of 38400 baud/s, 8 data bits, no parity, 1 stopbit. | ||
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If you have kept this default configuration, the line discipline can | ||
be attached on a command prompt as follows:: | ||
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sudo ldattach \ | ||
--debug \ | ||
--speed 38400 \ | ||
--eightbits \ | ||
--noparity \ | ||
--onestopbit \ | ||
--iflag -ICRNL,INLCR,-IXOFF \ | ||
30 \ | ||
/dev/ttyUSB0 | ||
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To change the ELM327's serial settings, please refer to its data | ||
sheet. This needs to be done before attaching the line discipline. | ||
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Once the ldisc is attached, the CAN interface starts out unconfigured. | ||
Set the speed before starting it:: | ||
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# The interface needs to be down to change parameters | ||
sudo ip link set can0 down | ||
sudo ip link set can0 type can bitrate 500000 | ||
sudo ip link set can0 up | ||
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500000 bit/s is a common rate for OBD-II diagnostics. | ||
If you're connecting straight to a car's OBD port, this is the speed | ||
that most cars (but not all!) expect. | ||
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After this, you can set out as usual with candump, cansniffer, etc. | ||
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How to check the controller version | ||
------------------------------------ | ||
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Use a terminal program to attach to the controller. | ||
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After issuing the "``AT WS``" command, the controller will respond with | ||
its version:: | ||
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>AT WS | ||
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ELM327 v1.4b | ||
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> | ||
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Note that clones may claim to be any version they like. | ||
It is not indicative of their actual feature set. | ||
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Communication example | ||
---------------------- | ||
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This is a short and incomplete introduction on how to talk to an ELM327. | ||
It is here to guide understanding of the controller's and the driver's | ||
limitation (listed below) as well as manual testing. | ||
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The ELM327 has two modes: | ||
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- Command mode | ||
- Reception mode | ||
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In command mode, it expects one command per line, terminated by CR. | ||
By default, the prompt is a "``>``", after which a command can be | ||
entered:: | ||
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>ATE1 | ||
OK | ||
> | ||
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The init script in the driver switches off several configuration options | ||
that are only meaningful in the original OBD scenario the chip is meant | ||
for, and are actually a hindrance for can327. | ||
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When a command is not recognized, such as by an older version of the | ||
ELM327, a question mark is printed as a response instead of OK:: | ||
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>ATUNKNOWN | ||
? | ||
> | ||
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At present, can327 does not evaluate this response. See the section | ||
below on known limitations for details. | ||
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When a CAN frame is to be sent, the target address is configured, after | ||
which the frame is sent as a command that consists of the data's hex | ||
dump:: | ||
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>ATSH123 | ||
OK | ||
>DEADBEEF12345678 | ||
OK | ||
> | ||
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The above interaction sends the SFF frame "``DE AD BE EF 12 34 56 78``" | ||
with (11 bit) CAN ID ``0x123``. | ||
For this to function, the controller must be configured for SFF sending | ||
mode (using "``AT PB``", see code or datasheet). | ||
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Once a frame has been sent and wait-for-reply mode is on (``ATR1``, | ||
configured on ``listen-only=off``), or when the reply timeout expires | ||
and the driver sets the controller into monitoring mode (``ATMA``), | ||
the ELM327 will send one line for each received CAN frame, consisting | ||
of CAN ID, DLC, and data:: | ||
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123 8 DEADBEEF12345678 | ||
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For EFF (29 bit) CAN frames, the address format is slightly different, | ||
which can327 uses to tell the two apart:: | ||
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12 34 56 78 8 DEADBEEF12345678 | ||
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The ELM327 will receive both SFF and EFF frames - the current CAN | ||
config (``ATPB``) does not matter. | ||
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If the ELM327's internal UART sending buffer runs full, it will abort | ||
the monitoring mode, print "BUFFER FULL" and drop back into command | ||
mode. Note that in this case, unlike with other error messages, the | ||
error message may appear on the same line as the last (usually | ||
incomplete) data frame:: | ||
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12 34 56 78 8 DEADBEEF123 BUFFER FULL | ||
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Known limitations of the controller | ||
------------------------------------ | ||
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- Clone devices ("v1.5" and others) | ||
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Sending RTR frames is not supported and will be dropped silently. | ||
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Receiving RTR with DLC 8 will appear to be a regular frame with | ||
the last received frame's DLC and payload. | ||
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"``AT CSM``" (CAN Silent Monitoring, i.e. don't send CAN ACKs) is | ||
not supported, and is hard coded to ON. Thus, frames are not ACKed | ||
while listening: "``AT MA``" (Monitor All) will always be "silent". | ||
However, immediately after sending a frame, the ELM327 will be in | ||
"receive reply" mode, in which it *does* ACK any received frames. | ||
Once the bus goes silent, or an error occurs (such as BUFFER FULL), | ||
or the receive reply timeout runs out, the ELM327 will end reply | ||
reception mode on its own and can327 will fall back to "``AT MA``" | ||
in order to keep monitoring the bus. | ||
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Other limitations may apply, depending on the clone and the quality | ||
of its firmware. | ||
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- All versions | ||
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No full duplex operation is supported. The driver will switch | ||
between input/output mode as quickly as possible. | ||
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The length of outgoing RTR frames cannot be set. In fact, some | ||
clones (tested with one identifying as "``v1.5``") are unable to | ||
send RTR frames at all. | ||
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We don't have a way to get real-time notifications on CAN errors. | ||
While there is a command (``AT CS``) to retrieve some basic stats, | ||
we don't poll it as it would force us to interrupt reception mode. | ||
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- Versions prior to 1.4b | ||
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These versions do not send CAN ACKs when in monitoring mode (AT MA). | ||
However, they do send ACKs while waiting for a reply immediately | ||
after sending a frame. The driver maximizes this time to make the | ||
controller as useful as possible. | ||
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Starting with version 1.4b, the ELM327 supports the "``AT CSM``" | ||
command, and the "listen-only" CAN option will take effect. | ||
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- Versions prior to 1.4 | ||
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These chips do not support the "``AT PB``" command, and thus cannot | ||
change bitrate or SFF/EFF mode on-the-fly. This will have to be | ||
programmed by the user before attaching the line discipline. See the | ||
data sheet for details. | ||
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- Versions prior to 1.3 | ||
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These chips cannot be used at all with can327. They do not support | ||
the "``AT D1``" command, which is necessary to avoid parsing conflicts | ||
on incoming data, as well as distinction of RTR frame lengths. | ||
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Specifically, this allows for easy distinction of SFF and EFF | ||
frames, and to check whether frames are complete. While it is possible | ||
to deduce the type and length from the length of the line the ELM327 | ||
sends us, this method fails when the ELM327's UART output buffer | ||
overruns. It may abort sending in the middle of the line, which will | ||
then be mistaken for something else. | ||
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Known limitations of the driver | ||
-------------------------------- | ||
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- No 8/7 timing. | ||
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ELM327 can only set CAN bitrates that are of the form 500000/n, where | ||
n is an integer divisor. | ||
However there is an exception: With a separate flag, it may set the | ||
speed to be 8/7 of the speed indicated by the divisor. | ||
This mode is not currently implemented. | ||
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- No evaluation of command responses. | ||
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The ELM327 will reply with OK when a command is understood, and with ? | ||
when it is not. The driver does not currently check this, and simply | ||
assumes that the chip understands every command. | ||
The driver is built such that functionality degrades gracefully | ||
nevertheless. See the section on known limitations of the controller. | ||
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- No use of hardware CAN ID filtering | ||
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An ELM327's UART sending buffer will easily overflow on heavy CAN bus | ||
load, resulting in the "``BUFFER FULL``" message. Using the hardware | ||
filters available through "``AT CF xxx``" and "``AT CM xxx``" would be | ||
helpful here, however SocketCAN does not currently provide a facility | ||
to make use of such hardware features. | ||
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Rationale behind the chosen configuration | ||
------------------------------------------ | ||
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``AT E1`` | ||
Echo on | ||
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We need this to be able to get a prompt reliably. | ||
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``AT S1`` | ||
Spaces on | ||
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We need this to distinguish 11/29 bit CAN addresses received. | ||
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Note: | ||
We can usually do this using the line length (odd/even), | ||
but this fails if the line is not transmitted fully to | ||
the host (BUFFER FULL). | ||
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``AT D1`` | ||
DLC on | ||
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We need this to tell the "length" of RTR frames. | ||
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A note on CAN bus termination | ||
------------------------------ | ||
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Your adapter may have resistors soldered in which are meant to terminate | ||
the bus. This is correct when it is plugged into a OBD-II socket, but | ||
not helpful when trying to tap into the middle of an existing CAN bus. | ||
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If communications don't work with the adapter connected, check for the | ||
termination resistors on its PCB and try removing them. |
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@@ -10,6 +10,7 @@ Contents: | |
.. toctree:: | ||
:maxdepth: 2 | ||
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can327 | ||
ctu/ctucanfd-driver | ||
freescale/flexcan | ||
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