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docs: Update CANBUS.md 

Recommend USB to CAN adapter and no longer recommend using the
waveshare rpi hat.

Recommend using allow-hotplug for all USB adapters.

Note bandwidth limitations when using USB to CAN bridge mode.

Note that a USB to CAN mcu is not a USB serial device.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
This commit is contained in:
Kevin O'Connor 2023-05-20 16:36:25 -04:00
parent 5f0d252b40
commit 81e4636f42
1 changed files with 44 additions and 37 deletions
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docs/CANBUS.md
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@ -4,7 +4,7 @@ This document describes Klipper's CAN bus support.
## Device Hardware
Klipper currently supports CAN on stm32, same5x, and rp2040 chips. In
Klipper currently supports CAN on stm32, SAME5x, and rp2040 chips. In
addition, the micro-controller chip must be on a board that has a CAN
transceiver.
@ -14,44 +14,35 @@ and flash it to the target board.
## Host Hardware
In order to use a CAN bus, it is necessary to have a host adapter.
There are currently two common options:
1. Use a
[Waveshare Raspberry Pi CAN hat](https://www.waveshare.com/rs485-can-hat.htm)
or one of its many clones.
2. Use a USB CAN adapter (for example
[https://hacker-gadgets.com/product/cantact-usb-can-adapter/](https://hacker-gadgets.com/product/cantact-usb-can-adapter/)). There
are many different USB to CAN adapters available - when choosing
one, we recommend verifying it can run the
[candlelight firmware](https://github.com/candle-usb/candleLight_fw).
(Unfortunately, we've found some USB adapters run defective
firmware and are locked down, so verify before purchasing.)
In order to use a CAN bus, it is necessary to have a host adapter. It
is recommended to use a "USB to CAN adapter". There are many different
USB to CAN adapters available from different manufacturers. When
choosing one, we recommend verifying that the firmware can be updated
on it. (Unfortunately, we've found some USB adapters run defective
firmware and are locked down, so verify before purchasing.) Look for
adapters that can run Klipper directly (in its "USB to CAN bridge
mode") or that run the
[candlelight firmware](https://github.com/candle-usb/candleLight_fw).
It is also necessary to configure the host operating system to use the
adapter. This is typically done by creating a new file named
`/etc/network/interfaces.d/can0` with the following contents:
```
auto can0
allow-hotplug can0
iface can0 can static
bitrate 500000
bitrate 1000000
up ifconfig $IFACE txqueuelen 128
```
Note that the "Raspberry Pi CAN hat" also requires
[changes to config.txt](https://www.waveshare.com/wiki/RS485_CAN_HAT).
## Terminating Resistors
A CAN bus should have two 120 ohm resistors between the CANH and CANL
wires. Ideally, one resistor located at each the end of the bus.
Note that some devices have a builtin 120 ohm resistor (for example,
the "Waveshare Raspberry Pi CAN hat" has a soldered on resistor that
can not be easily removed). Some devices do not include a resistor at
all. Other devices have a mechanism to select the resistor (typically
by connecting a "pin jumper"). Be sure to check the schematics of all
Note that some devices have a builtin 120 ohm resistor that can not be
easily removed. Some devices do not include a resistor at all. Other
devices have a mechanism to select the resistor (typically by
connecting a "pin jumper"). Be sure to check the schematics of all
devices on the CAN bus to verify that there are two and only two 120
Ohm resistors on the bus.
@ -95,22 +86,18 @@ canbus_uuid: 11aa22bb33cc
## USB to CAN bus bridge mode
Some micro-controllers support selecting "USB to CAN bus bridge" mode
during "make menuconfig". This mode may allow one to use a
during Klipper's "make menuconfig". This mode may allow one to use a
micro-controller as both a "USB to CAN bus adapter" and as a Klipper
node.
When Klipper uses this mode the micro-controller appears as a "USB CAN
bus adapter" under Linux. The "Klipper bridge mcu" itself will appear
as if was on this CAN bus - it can be identified via `canbus_query.py`
and configured like other CAN bus Klipper nodes. It will appear
alongside other devices that are actually on the CAN bus.
as if it was on this CAN bus - it can be identified via
`canbus_query.py` and it must be configured like other CAN bus Klipper
nodes.
Some important notes when using this mode:
* The "bridge mcu" is not actually on the CAN bus. Messages to and
from it do not consume bandwidth on the CAN bus. The mcu can not be
seen by other adapters that may be on the CAN bus.
* It is necessary to configure the `can0` (or similar) interface in
Linux in order to communicate with the bus. However, Linux CAN bus
speed and CAN bus bit-timing options are ignored by Klipper.
@ -119,12 +106,32 @@ Some important notes when using this mode:
* Whenever the "bridge mcu" is reset, Linux will disable the
corresponding `can0` interface. To ensure proper handling of
FIRMWARE_RESTART and RESTART commands, it is recommended to replace
`auto` with `allow-hotplug` in the `/etc/network/interfaces.d/can0`
file. For example:
FIRMWARE_RESTART and RESTART commands, it is recommended to use
`allow-hotplug` in the `/etc/network/interfaces.d/can0` file. For
example:
```
allow-hotplug can0
iface can0 can static
bitrate 500000
bitrate 1000000
up ifconfig $IFACE txqueuelen 128
```
* The "bridge mcu" is not actually on the CAN bus. Messages to and
from the bridge mcu will not be seen by other adapters that may be
on the CAN bus.
* The available bandwidth to both the "bridge mcu" itself and all
devices on the CAN bus is effectively limited by the CAN bus
frequency. As a result, it is recommended to use a CAN bus frequency
of 1000000 when using "USB to CAN bus bridge mode".
Even at a CAN bus frequency of 1000000, there may not be sufficient
bandwidth to run a `SHAPER_CALIBRATE` test if both the XY steppers
and the accelerometer all communicate via a single "USB to CAN bus"
interface.
* A USB to CAN bridge board will not appear as a USB serial device, it
will not show up when running `ls /dev/serial/by-id`, and it can not
be configured in Klipper's printer.cfg file with a `serial:`
parameter. The bridge board appears as a "USB CAN adapter" and it is
configured in the printer.cfg as a [CAN node](#configuring-klipper).