w1: add UART w1 bus driver

[linux-2.6-block.git] / Documentation / w1 / masters / w1-uart.rst

.. SPDX-License-Identifier: GPL-2.0-or-later

=====================
Kernel driver w1-uart
=====================

Author: Christoph Winklhofer <cj.winklhofer@gmail.com>


Description
-----------

UART 1-Wire bus driver. The driver utilizes the UART interface via the
Serial Device Bus to create the 1-Wire timing patterns as described in
the document `"Using a UART to Implement a 1-Wire Bus Master"`_.

.. _"Using a UART to Implement a 1-Wire Bus Master": https://www.analog.com/en/technical-articles/using-a-uart-to-implement-a-1wire-bus-master.html

In short, the UART peripheral must support full-duplex and operate in
open-drain mode. The timing patterns are generated by a specific
combination of baud-rate and transmitted byte, which corresponds to a
1-Wire read bit, write bit or reset pulse.

For instance the timing pattern for a 1-Wire reset and presence detect uses
the baud-rate 9600, i.e. 104.2 us per bit. The transmitted byte 0xf0 over
UART (least significant bit first, start-bit low) sets the reset low time
for 1-Wire to 521 us. A present 1-Wire device changes the received byte by
pulling the line low, which is used by the driver to evaluate the result of
the 1-Wire operation.

Similar for a 1-Wire read bit or write bit, which uses the baud-rate
115200, i.e. 8.7 us per bit. The transmitted byte 0x80 is used for a
Write-0 operation (low time 69.6us) and the byte 0xff for Read-0, Read-1
and Write-1 (low time 8.7us).

The default baud-rate for reset and presence detection is 9600 and for
a 1-Wire read or write operation 115200. In case the actual baud-rate
is different from the requested one, the transmitted byte is adapted
to generate the 1-Wire timing patterns.


Usage
-----

Specify the UART 1-wire bus in the device tree by adding the single child
onewire to the serial node (e.g. uart0). For example:
::

  @uart0 {
    ...
    onewire {
      compatible = "w1-uart";
    };
  };