René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`brp` parameter validation for `BitTiming` can be too restrictive for some peripherals","articleBody":"### Describe the bug\nThe `BitTiming` class unconditionally requires that the bitrate prescaler `brp` is within the interval `[1,64]`; this is checked in the `_validate` function, which is unconditionally called from `__init__` even when `strict=False`:\n```python\n    def _validate(self) -\u003e None:\n        if not 1 \u003c= self.brp \u003c= 64:\n            raise ValueError(f\"bitrate prescaler (={self.brp}) must be in [1...64].\")\n\n        # other checks omitted\n```\n\nI was using `python-can` with a FYSETC UCAN transceiver running candleLight firmware, which speaks the `gs_usb` protocol. The transceiver uses a clock frequency of `fclock=48MHz`, but it also supports a large range of possible `brp` values from 1 to 1024 to implement lower bitrates (see [bxcan.c](https://github.com/candle-usb/candleLight_fw/blob/5fc2e6b560928f94079e206364354250157448c2/src/can/bxcan.c#L57) ). \n\nI need to configure a baud rate of 33333 bit/s and want to use the default sample point of 87.5%, which requires a prescaler value \u003e 64 to keep the other paramters (specifically `tseg1` within their valid range). This is currently not possible without patching the code of `python-can` directly, since the `gs_usb` interface kind does not allow direct specification of the `BitTimings`, and the validation check will reject the prescaler value, so the automatic parameter finding does not work. However, the device supports higher prescaler values just fine; and removing the bitrate prescaler check in `_validate` allows me to configure the device and send CAN messages.\n\n\n\n### To Reproduce\nThe following code will raise `ValueError(\"No suitable bit timings found.\")` from `BitTimings.from_sample_point`\n```python\nimport can\ncan_bus = can.Bus(interface='gs_usb', channel=0, bitrate=33333)\n```\n\nNote: this requires a suitable board with candleLight firmware to be present and attached. Internally, the device configuration routine retrieves the CAN clock speed and calls `BitTiming.from_sample_point` with those values:\n\n```python\nfrom can.bit_timing import BitTiming\nBitTiming.from_sample_point(f_clock=48000000,bitrate=33333,sample_point=87.5)\n```\nThis can be called without hardware to reproduce the issue.\n\n### Expected behavior\nIdeally, the parameter validation takes the valid parameter range of the attached device into account (if known), so that the code above just works as-is. The `gs_usb` protocol allows the host to query valid timing parameters, including the CAN clock frequency and the range of allowed `brp` values (start, stop, increment). \n\nAlternatively, having an option to pass a raw `BitTimings` object to the `can.Bus` method would work as well, although it would be less convenient.\n\n### Additional context\n\nOS and version: Linux 6.17.0-23-generic (Ubuntu 25.10)\nPython version: 3.13.7\npython-can version: 4.6.1\npython-can interface/s (if applicable): gs_usb\n\n\n\n\u003cdetails\u003e\u003csummary\u003eTraceback and logs\u003c/summary\u003e\n\n```\nTraceback (most recent call last):\n  File \"/home/user/Development/canbus/./gs_usb_bitrate_test.py\", line 3, in \u003cmodule\u003e\n    can_bus = can.Bus(interface='gs_usb', channel=0, bitrate=33333)\n  File \"/home/user/Development/canbus/venv/lib/python3.13/site-packages/can/util.py\", line 392, in wrapper\n    return f(*args, **kwargs)\n  File \"/home/user/Development/canbus/venv/lib/python3.13/site-packages/can/interface.py\", line 137, in Bus\n    bus = cls(channel, **kwargs)\n  File \"/home/user/Development/canbus/venv/lib/python3.13/site-packages/can/interfaces/gs_usb.py\", line 63, in __init__\n    bit_timing = can.BitTiming.from_sample_point(\n        f_clock=self.gs_usb.device_capability.fclk_can,\n        bitrate=bitrate,\n        sample_point=87.5,\n    )\n  File \"/home/user/Development/canbus/venv/lib/python3.13/site-packages/can/bit_timing.py\", line 295, in from_sample_point\n    raise ValueError(\"No suitable bit timings found.\")\nValueError: No suitable bit timings found.\n```\n\n\u003c/details\u003e\n","author":{"url":"https://github.com/SeTcbPrivilege","@type":"Person","name":"SeTcbPrivilege"},"datePublished":"2026-05-15T15:30:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/2061/python-can/issues/2061"}