1 ===================== 1 =====================
2 I2C Ten-bit Addresses 2 I2C Ten-bit Addresses
3 ===================== 3 =====================
4 4
5 The I2C protocol knows about two kinds of devi 5 The I2C protocol knows about two kinds of device addresses: normal 7 bit
6 addresses, and an extended set of 10 bit addre 6 addresses, and an extended set of 10 bit addresses. The sets of addresses
7 do not intersect: the 7 bit address 0x10 is no 7 do not intersect: the 7 bit address 0x10 is not the same as the 10 bit
8 address 0x10 (though a single device could res 8 address 0x10 (though a single device could respond to both of them).
9 To avoid ambiguity, the user sees 10 bit addre 9 To avoid ambiguity, the user sees 10 bit addresses mapped to a different
10 address space, namely 0xa000-0xa3ff. The leadi 10 address space, namely 0xa000-0xa3ff. The leading 0xa (= 10) represents the
11 10 bit mode. This is used for creating device 11 10 bit mode. This is used for creating device names in sysfs. It is also
12 needed when instantiating 10 bit devices via t 12 needed when instantiating 10 bit devices via the new_device file in sysfs.
13 13
14 I2C messages to and from 10-bit address device 14 I2C messages to and from 10-bit address devices have a different format.
15 See the I2C specification for the details. 15 See the I2C specification for the details.
16 16
17 The current 10 bit address support is minimal. 17 The current 10 bit address support is minimal. It should work, however
18 you can expect some problems along the way: 18 you can expect some problems along the way:
19 19
20 * Not all bus drivers support 10-bit addresses 20 * Not all bus drivers support 10-bit addresses. Some don't because the
21 hardware doesn't support them (SMBus doesn't 21 hardware doesn't support them (SMBus doesn't require 10-bit address
22 support for example), some don't because nob 22 support for example), some don't because nobody bothered adding the
23 code (or it's there but not working properly 23 code (or it's there but not working properly.) Software implementation
24 (i2c-algo-bit) is known to work. 24 (i2c-algo-bit) is known to work.
25 * Some optional features do not support 10-bit 25 * Some optional features do not support 10-bit addresses. This is the
26 case of automatic detection and instantiatio 26 case of automatic detection and instantiation of devices by their,
27 drivers, for example. 27 drivers, for example.
28 * Many user-space packages (for example i2c-to 28 * Many user-space packages (for example i2c-tools) lack support for
29 10-bit addresses. 29 10-bit addresses.
30 30
31 Note that 10-bit address devices are still pre 31 Note that 10-bit address devices are still pretty rare, so the limitations
32 listed above could stay for a long time, maybe 32 listed above could stay for a long time, maybe even forever if nobody
33 needs them to be fixed. 33 needs them to be fixed.
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