i2cdetect
Detect I2C devices on a bus
TLDR
List active I2C buses
Scan devices on an I2C bus
Scan devices on an I2C bus without asking for confirmation
SYNOPSIS
i2cdetect [options] I2CBUS
i2cdetect -l
PARAMETERS
-l, --list
List all detected I2C/SMBus adapters with bus numbers.
-y I2CBUS, --yes-do-it
Disable interactive prompt; force scan on specified bus.
-r, --read-also
Probe with SMBus 'read byte' protocol too (slower, for non-standard devices).
-q, --quiet
Suppress non-fatal warnings, like 10-bit address notes.
-a
Probe full address range 0x00-0x7f (default excludes some reserved).
-V, --version
Print version information and exit.
I2CBUS
Bus number to scan, e.g. 1 for /dev/i2c-1.
DESCRIPTION
i2cdetect is a utility from the i2c-tools package for scanning I2C (Inter-Integrated Circuit) or SMBus buses to detect attached slave devices. It probes each 7-bit address (default range 0x03-0x77) by sending a dummy start/stop sequence and checking for ACK responses.
The output is a grid table:
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: 50 -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
Numbers indicate responding addresses, UU shows kernel-bound drivers, -- means no ACK.
Essential for debugging hardware on embedded systems like Raspberry Pi, Arduino shields, or industrial controllers. Interactive mode prompts 'Scan bus?'; use -y for non-interactive scripting. List buses with -l. Requires access to /dev/i2c-* (root or i2c group).
Probes are quick but -r adds SMBus read-byte for stubborn devices, increasing scan time. Widely used since early 2000s for verifying sensors, EEPROMs, RTCs.
CAVEATS
Probing may lock up or reset sensitive hardware; avoid on production systems. Interactive mode prevents accidents—do not always use -y. Some devices ignore probes or share addresses. Requires kernel I2C support and device permissions.
OUTPUT LEGEND
--: No response.
XX: Address acknowledged (e.g. 50 = 0x50).
UU: Kernel driver bound (instantiate_info).
Whitespace: Not probed.
PERMISSIONS
Needs read/write on /dev/i2c-BUS. Add user to i2c group: sudo usermod -aG i2c $USER.
EXAMPLE
i2cdetect -l → lists buses.
i2cdetect -y 1 → scans bus 1 non-interactively.
HISTORY
Introduced in early i2c-tools (circa 2001) for Linux 2.4/2.6 I2C subsystem by Frodo Looijaard and Mark D. Studebaker. Maintained by Jean Delvare; evolved with SMBus support and kernel driver integration.
