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diff --git a/Documentation/i2c/ten-bit-addresses.rst b/Documentation/i2c/ten-bit-addresses.rst new file mode 100644 index 000000000..5c765aff1 --- /dev/null +++ b/Documentation/i2c/ten-bit-addresses.rst @@ -0,0 +1,33 @@ +===================== +I2C Ten-bit Addresses +===================== + +The I2C protocol knows about two kinds of device addresses: normal 7 bit +addresses, and an extended set of 10 bit addresses. The sets of addresses +do not intersect: the 7 bit address 0x10 is not the same as the 10 bit +address 0x10 (though a single device could respond to both of them). +To avoid ambiguity, the user sees 10 bit addresses mapped to a different +address space, namely 0xa000-0xa3ff. The leading 0xa (= 10) represents the +10 bit mode. This is used for creating device names in sysfs. It is also +needed when instantiating 10 bit devices via the new_device file in sysfs. + +I2C messages to and from 10-bit address devices have a different format. +See the I2C specification for the details. + +The current 10 bit address support is minimal. It should work, however +you can expect some problems along the way: + +* Not all bus drivers support 10-bit addresses. Some don't because the + hardware doesn't support them (SMBus doesn't require 10-bit address + support for example), some don't because nobody bothered adding the + code (or it's there but not working properly.) Software implementation + (i2c-algo-bit) is known to work. +* Some optional features do not support 10-bit addresses. This is the + case of automatic detection and instantiation of devices by their, + drivers, for example. +* Many user-space packages (for example i2c-tools) lack support for + 10-bit addresses. + +Note that 10-bit address devices are still pretty rare, so the limitations +listed above could stay for a long time, maybe even forever if nobody +needs them to be fixed. |