hwmon: (ds1621) Clean up documentation

readings. Be sure to have a high vs. low temperature limit gap of al least

1.0 degree Celsius to avoid Tout "bouncing", though!

As for alarms, you can read the alarm status of the DS1621 via the 'alarms'

/sys file interface. The result consists mainly of bit 6 and 5 of the

configuration register of the chip; bit 6 (0x40 or 64) is the high alarm

bit and bit 5 (0x20 or 32) the low one. These bits are set when the high or

low limits are met or exceeded and are reset by the module as soon as the

respective temperature ranges are left.

The alarm bits are set when the high or low limits are met or exceeded and

are reset by the module as soon as the respective temperature ranges are

left.

The alarm registers are in no way suitable to find out about the actual

status of Tout. They will only tell you about its history, whether or not

Temperature conversion of the DS1621 takes up to 1000ms; internal access to

non-volatile registers may last for 10ms or below.

High Accuracy Temperature Reading

---------------------------------

As said before, the temperature issued via the 9-bit i2c-bus data is

somewhat arbitrary. Internally, the temperature conversion is of a

different kind that is explained (not so...) well in the DS1621 data sheet.

To cut the long story short: Inside the DS1621 there are two oscillators,

both of them biassed by a temperature coefficient.

Higher resolution of the temperature reading can be achieved using the

internal projection, which means taking account of REG_COUNT and REG_SLOPE

(the driver manages them):

Taken from Dallas Semiconductors App Note 068: 'Increasing Temperature

Resolution on the DS1620' and App Note 105: 'High Resolution Temperature

Measurement with Dallas Direct-to-Digital Temperature Sensors'

- Read the 9-bit temperature and strip the LSB (Truncate the .5 degs)

- The resulting value is TEMP_READ.

- Then, read REG_COUNT.

- And then, REG_SLOPE.

      TEMP = TEMP_READ - 0.25 + ((REG_SLOPE - REG_COUNT) / REG_SLOPE)

Note that this is what the DONE bit in the DS1621 configuration register is

good for: Internally, one temperature conversion takes up to 1000ms. Before

that conversion is complete you will not be able to read valid things out

of REG_COUNT and REG_SLOPE. The DONE bit, as you may have guessed by now,

tells you whether the conversion is complete ("done", in plain English) and

thus, whether the values you read are good or not.

The DS1621 has two modes of operation: "Continuous" conversion, which can

be understood as the default stand-alone mode where the chip gets the

temperature and controls external devices via its Tout pin or tells other

i2c's about it if they care. The other mode is called "1SHOT", that means

that it only figures out about the temperature when it is explicitly told

to do so; this can be seen as power saving mode.

Now if you want to read REG_COUNT and REG_SLOPE, you have to either stop

the continuous conversions until the contents of these registers are valid,

or, in 1SHOT mode, you have to have one conversion made.