138 lines
5.2 KiB
ReStructuredText
138 lines
5.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Kernel driver max31827
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======================
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Supported chips:
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* Maxim MAX31827
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Prefix: 'max31827'
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Addresses scanned: I2C 0x40 - 0x5f
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Datasheet: Publicly available at the Analog Devices website
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* Maxim MAX31828
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Prefix: 'max31828'
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Addresses scanned: I2C 0x40 - 0x5f
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Datasheet: Publicly available at the Analog Devices website
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* Maxim MAX31829
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Prefix: 'max31829'
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Addresses scanned: I2C 0x40 - 0x5f
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Datasheet: Publicly available at the Analog Devices website
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Authors:
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- Daniel Matyas <daniel.matyas@analog.com>
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Description
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-----------
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The chips supported by this driver are quite similar. The only difference
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between them is found in the default power-on behaviour of the chips. While the
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MAX31827's fault queue is set to 1, the other two chip's fault queue is set to
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4. Besides this, the MAX31829's alarm active state is high, while the other two
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chip's alarms are active on low. It is important to note that the chips can be
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configured to operate in the same manner with 1 write operation to the
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configuration register. From here on, we will refer to all these chips as
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MAX31827.
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MAX31827 implements a temperature sensor with a 6 WLP packaging scheme. This
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sensor measures the temperature of the chip itself.
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MAX31827 has low and over temperature alarms with an effective value and a
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hysteresis value: -40 and -30 degrees for under temperature alarm and +100 and
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+90 degrees for over temperature alarm.
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The alarm can be configured in comparator and interrupt mode from the
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devicetree. In Comparator mode, the OT/UT status bits have a value of 1 when the
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temperature rises above the TH value or falls below TL, which is also subject to
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the Fault Queue selection. OT status returns to 0 when the temperature drops
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below the TH_HYST value or when shutdown mode is entered. Similarly, UT status
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returns to 0 when the temperature rises above TL_HYST value or when shutdown
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mode is entered.
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In interrupt mode exceeding TH also sets OT status to 1, which remains set until
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a read operation is performed on the configuration/status register (max or min
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attribute); at this point, it returns to 0. Once OT status is set to 1 from
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exceeding TH and reset, it is set to 1 again only when the temperature drops
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below TH_HYST. The output remains asserted until it is reset by a read. It is
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set again if the temperature rises above TH, and so on. The same logic applies
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to the operation of the UT status bit.
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Putting the MAX31827 into shutdown mode also resets the OT/UT status bits. Note
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that if the mode is changed while OT/UT status bits are set, an OT/UT status
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reset may be required before it begins to behave normally. To prevent this,
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it is recommended to perform a read of the configuration/status register to
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clear the status bits before changing the operating mode.
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The conversions can be manual with the one-shot functionality and automatic with
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a set frequency. When powered on, the chip measures temperatures with 1 conv/s.
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The conversion rate can be modified with update_interval attribute of the chip.
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Conversion/second = 1/update_interval. Thus, the available options according to
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the data sheet are:
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- 64000 (ms) = 1 conv/64 sec
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- 32000 (ms) = 1 conv/32 sec
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- 16000 (ms) = 1 conv/16 sec
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- 4000 (ms) = 1 conv/4 sec
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- 1000 (ms) = 1 conv/sec (default)
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- 250 (ms) = 4 conv/sec
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- 125 (ms) = 8 conv/sec
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Enabling the device when it is already enabled has the side effect of setting
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the conversion frequency to 1 conv/s. The conversion time varies depending on
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the resolution.
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The conversion time doubles with every bit of increased resolution. The
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available resolutions are:
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- 8 bit -> 8.75 ms conversion time
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- 9 bit -> 17.5 ms conversion time
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- 10 bit -> 35 ms conversion time
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- 12 bit (default) -> 140 ms conversion time
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There is a temp1_resolution attribute which indicates the unit change in the
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input temperature in milli-degrees C.
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- 1000 mC -> 8 bit
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- 500 mC -> 9 bit
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- 250 mC -> 10 bit
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- 62 mC -> 12 bit (default) - actually this is 62.5, but the fil returns 62
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When chip is in shutdown mode and a read operation is requested, one-shot is
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triggered, the device waits for <conversion time> ms, and only after that is
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the temperature value register read. Note that the conversion times are rounded
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up to the nearest possible integer.
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The LSB of the temperature values is 0.0625 degrees Celsius, but the values of
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the temperatures are displayed in milli-degrees. This means, that some data is
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lost. The step between 2 consecutive values is 62 or 63. This effect can be seen
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in the writing of alarm values too. For positive numbers the user-input value
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will always be rounded down to the nearest possible value, for negative numbers
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the user-input will always be rounded up to the nearest possible value.
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Bus timeout resets the I2C-compatible interface when SCL is low for more than
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30ms (nominal).
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Alarm polarity determines if the active state of the alarm is low or high. The
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behavior for both settings is dependent on the Fault Queue setting. The ALARM
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pin is an open-drain output and requires a pullup resistor to operate.
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The Fault Queue bits select how many consecutive temperature faults must occur
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before overtemperature or undertemperature faults are indicated in the
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corresponding status bits.
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Notes
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-----
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PEC is not implemented.
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