.. SPDX-License-Identifier: GPL-2.0

Kernel driver max31827
======================

Supported chips:

  * Maxim MAX31827

    Prefix: 'max31827'

    Addresses scanned: I2C 0x40 - 0x5f

    Datasheet: Publicly available at the Analog Devices website

  * Maxim MAX31828

    Prefix: 'max31828'

    Addresses scanned: I2C 0x40 - 0x5f

    Datasheet: Publicly available at the Analog Devices website

  * Maxim MAX31829

    Prefix: 'max31829'

    Addresses scanned: I2C 0x40 - 0x5f

    Datasheet: Publicly available at the Analog Devices website


Authors:
	- Daniel Matyas <daniel.matyas@analog.com>

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

The chips supported by this driver are quite similar. The only difference
between them is found in the default power-on behaviour of the chips. While the
MAX31827's fault queue is set to 1, the other two chip's fault queue is set to
4. Besides this, the MAX31829's alarm active state is high, while the other two
chip's alarms are active on low. It is important to note that the chips can be
configured to operate in the same manner with 1 write operation to the
configuration register. From here on, we will refer to all these chips as
MAX31827.

MAX31827 implements a temperature sensor with a 6 WLP packaging scheme. This
sensor measures the temperature of the chip itself.

MAX31827 has low and over temperature alarms with an effective value and a
hysteresis value: -40 and -30 degrees for under temperature alarm and +100 and
+90 degrees for over temperature alarm.

The alarm can be configured in comparator and interrupt mode. Currently only
comparator mode is implemented. In Comparator mode, the OT/UT status bits have a
value of 1 when the temperature rises above the TH value or falls below TL,
which is also subject to the Fault Queue selection. OT status returns to 0 when
the temperature drops below the TH_HYST value or when shutdown mode is entered.
Similarly, UT status returns to 0 when the temperature rises above TL_HYST value
or when shutdown mode is entered.

Putting the MAX31827 into shutdown mode also resets the OT/UT status bits. Note
that if the mode is changed while OT/UT status bits are set, an OT/UT status
reset may be required before it begins to behave normally. To prevent this,
it is recommended to perform a read of the configuration/status register to
clear the status bits before changing the operating mode.

The conversions can be manual with the one-shot functionality and automatic with
a set frequency. When powered on, the chip measures temperatures with 1 conv/s.
Enabling the device when it is already enabled has the side effect of setting
the conversion frequency to 1 conv/s. The conversion time varies depending on
the resolution. The conversion time doubles with every bit of increased
resolution. For 10 bit resolution 35ms are needed, while for 12 bit resolution
(default) 140ms. When chip is in shutdown mode and a read operation is
requested, one-shot is triggered, the device waits for 140 (conversion time) ms,
and only after that is the temperature value register read.

The LSB of the temperature values is 0.0625 degrees Celsius, but the values of
the temperatures are displayed in milli-degrees. This means, that some data is
lost. The step between 2 consecutive values is 62 or 63. This effect can be seen
in the writing of alarm values too. For positive numbers the user-input value
will always be rounded down to the nearest possible value, for negative numbers
the user-input will always be rounded up to the nearest possible value.

Notes
-----

Currently fault queue, alarm polarity and resolution cannot be modified.
PEC is not implemented either.