FAQ

Short answers to questions asked frequently in the issue tracker.

Does smbus2 work on Windows or macOS?

No. smbus2 requires the Linux kernel’s I2C character device subsystem (/dev/i2c-*) and imports the Linux-only fcntl module. Neither macOS nor Windows provides these. See #73 and Platform Support.

Can I read or write more than 32 bytes at once?

Not with the standard SMBus block commands (read_i2c_block_data, write_i2c_block_data), which are capped at 32 bytes by the Linux SMBus implementation. Use i2c_rdwr with i2c_msg objects for larger transfers — there is no fixed limit at the smbus2 level for i2c_rdwr. See #35, #67, #99, and Combined Transactions with i2c_rdwr.

How do I perform a repeated start (write then read without STOP)?

Use bus.i2c_rdwr(write_msg, read_msg) where both messages are i2c_msg objects. A single ioctl call is made; the kernel inserts a repeated-start condition between the two messages. See #25 and Write-Then-Read (Repeated Start).

Can I use 10-bit I2C addresses?

smbus2 does not provide a dedicated 10-bit address API. You can check whether the adapter supports 10-bit addressing with bus.funcs & I2cFunc.ADDR_10BIT, but there is no built-in convenience method for opening or sending to 10-bit addresses. See #54.

Why does my device not respond when I use `read_byte_data` or `write_byte_data`?

The *_data functions always prepend a write of the register/offset byte to every transaction. Devices that do not implement the SMBus register-addressing model are confused by this extra write and may NAK the address or return wrong data.

Use i2c_rdwr with bare i2c_msg objects for such devices, or write_byte / read_byte for single-byte commands. See #19, #84, #117, and Reading Devices That Have No Register Address.

Can I use smbus2 with asyncio?

Not directly — smbus2 is synchronous. Options:

Wrap individual calls with loop.run_in_executor.
Use the community library smbus2_asyncio: https://github.com/jabdoa2/smbus2_asyncio

How do I set the I2C clock speed?

smbus2 does not control the clock speed. Configure it via the Linux kernel — for example, through a device-tree overlay on Raspberry Pi, or a kernel module parameter. See #77 and Setting I2C Bus Clock Speed (Baud Rate).

Can I open multiple `SMBus` instances for the same bus in one process?

It is not recommended. Multiple open file descriptors on the same /dev/i2c-N can lead to unexpected errors. Share a single SMBus instance across your code; use a threading.Lock if accessed from multiple threads. See #75 and Do Not Create Multiple SMBus Instances for the Same Bus.

How do I convert a raw unsigned value to a signed integer?

Use Python’s ctypes:

from ctypes import c_int8
signed = c_int8(bus.read_byte_data(addr, reg)).value

See Signed Integer Conversion for the full recipe including 16-bit words.

Where is the API documentation?

Full API documentation is available in the API Reference section of this documentation, and hosted online at https://smbus2.readthedocs.io/en/latest/.