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R.Marek@sh.cvut.cz7f15b662005-05-26 12:42:19 +00001Kernel driver ds1621
2====================
3
4Supported chips:
5 * Dallas Semiconductor DS1621
6 Prefix: 'ds1621'
7 Addresses scanned: I2C 0x48 - 0x4f
8 Datasheet: Publicly available at the Dallas Semiconductor website
9 http://www.dalsemi.com/
10 * Dallas Semiconductor DS1625
11 Prefix: 'ds1621'
12 Addresses scanned: I2C 0x48 - 0x4f
13 Datasheet: Publicly available at the Dallas Semiconductor website
14 http://www.dalsemi.com/
15
16Authors:
17 Christian W. Zuckschwerdt <zany@triq.net>
18 valuable contributions by Jan M. Sendler <sendler@sendler.de>
19 ported to 2.6 by Aurelien Jarno <aurelien@aurel32.net>
20 with the help of Jean Delvare <khali@linux-fr.org>
21
22Module Parameters
23------------------
24
25* polarity int
26 Output's polarity: 0 = active high, 1 = active low
27
28Description
29-----------
30
31The DS1621 is a (one instance) digital thermometer and thermostat. It has
32both high and low temperature limits which can be user defined (i.e.
33programmed into non-volatile on-chip registers). Temperature range is -55
34degree Celsius to +125 in 0.5 increments. You may convert this into a
35Fahrenheit range of -67 to +257 degrees with 0.9 steps. If polarity
36parameter is not provided, original value is used.
37
38As for the thermostat, behavior can also be programmed using the polarity
39toggle. On the one hand ("heater"), the thermostat output of the chip,
40Tout, will trigger when the low limit temperature is met or underrun and
41stays high until the high limit is met or exceeded. On the other hand
42("cooler"), vice versa. That way "heater" equals "active low", whereas
43"conditioner" equals "active high". Please note that the DS1621 data sheet
44is somewhat misleading in this point since setting the polarity bit does
45not simply invert Tout.
46
47A second thing is that, during extensive testing, Tout showed a tolerance
48of up to +/- 0.5 degrees even when compared against precise temperature
49readings. Be sure to have a high vs. low temperature limit gap of al least
501.0 degree Celsius to avoid Tout "bouncing", though!
51
52As for alarms, you can read the alarm status of the DS1621 via the 'alarms'
53/sys file interface. The result consists mainly of bit 6 and 5 of the
54configuration register of the chip; bit 6 (0x40 or 64) is the high alarm
55bit and bit 5 (0x20 or 32) the low one. These bits are set when the high or
56low limits are met or exceeded and are reset by the module as soon as the
57respective temperature ranges are left.
58
59The alarm registers are in no way suitable to find out about the actual
60status of Tout. They will only tell you about its history, whether or not
61any of the limits have ever been met or exceeded since last power-up or
62reset. Be aware: When testing, it showed that the status of Tout can change
63with neither of the alarms set.
64
65Temperature conversion of the DS1621 takes up to 1000ms; internal access to
66non-volatile registers may last for 10ms or below.
67
68High Accuracy Temperature Reading
69---------------------------------
70
71As said before, the temperature issued via the 9-bit i2c-bus data is
72somewhat arbitrary. Internally, the temperature conversion is of a
73different kind that is explained (not so...) well in the DS1621 data sheet.
74To cut the long story short: Inside the DS1621 there are two oscillators,
75both of them biassed by a temperature coefficient.
76
77Higher resolution of the temperature reading can be achieved using the
78internal projection, which means taking account of REG_COUNT and REG_SLOPE
79(the driver manages them):
80
81Taken from Dallas Semiconductors App Note 068: 'Increasing Temperature
82Resolution on the DS1620' and App Note 105: 'High Resolution Temperature
83Measurement with Dallas Direct-to-Digital Temperature Sensors'
84
85- Read the 9-bit temperature and strip the LSB (Truncate the .5 degs)
86- The resulting value is TEMP_READ.
87- Then, read REG_COUNT.
88- And then, REG_SLOPE.
89
90 TEMP = TEMP_READ - 0.25 + ((REG_SLOPE - REG_COUNT) / REG_SLOPE)
91
92Note that this is what the DONE bit in the DS1621 configuration register is
93good for: Internally, one temperature conversion takes up to 1000ms. Before
94that conversion is complete you will not be able to read valid things out
95of REG_COUNT and REG_SLOPE. The DONE bit, as you may have guessed by now,
96tells you whether the conversion is complete ("done", in plain English) and
97thus, whether the values you read are good or not.
98
99The DS1621 has two modes of operation: "Continuous" conversion, which can
100be understood as the default stand-alone mode where the chip gets the
101temperature and controls external devices via its Tout pin or tells other
102i2c's about it if they care. The other mode is called "1SHOT", that means
103that it only figures out about the temperature when it is explicitly told
104to do so; this can be seen as power saving mode.
105
106Now if you want to read REG_COUNT and REG_SLOPE, you have to either stop
107the continuous conversions until the contents of these registers are valid,
108or, in 1SHOT mode, you have to have one conversion made.