Module Parameters¶

• polarity intOutput’s polarity:
  • 0 = active high,
  • 1 = active low

Description¶

The DS1621 is a (one instance) digital thermometer and thermostat. It hasboth high and low temperature limits which can be user defined (i.e.programmed into non-volatile on-chip registers). Temperature range is -55degree Celsius to +125 in 0.5 increments. You may convert this into aFahrenheit range of -67 to +257 degrees with 0.9 steps. If polarityparameter is not provided, original value is used.

As for the thermostat, behavior can also be programmed using the polaritytoggle. On the one hand (“heater”), the thermostat output of the chip,Tout, will trigger when the low limit temperature is met or underrun andstays high until the high limit is met or exceeded. On the other hand(“cooler”), vice versa. That way “heater” equals “active low”, whereas“conditioner” equals “active high”. Please note that the DS1621 data sheetis somewhat misleading in this point since setting the polarity bit doesnot simply invert Tout.

A second thing is that, during extensive testing, Tout showed a toleranceof up to +/- 0.5 degrees even when compared against precise temperaturereadings. Be sure to have a high vs. low temperature limit gap of al least1.0 degree Celsius to avoid Tout “bouncing”, though!

The alarm bits are set when the high or low limits are met or exceeded andare reset by the module as soon as the respective temperature ranges areleft.

The alarm registers are in no way suitable to find out about the actualstatus of Tout. They will only tell you about its history, whether or notany of the limits have ever been met or exceeded since last power-up orreset. Be aware: When testing, it showed that the status of Tout can changewith neither of the alarms set.

Since there is no version or vendor identification register, there isno unique identification for these devices. Therefore, explicit deviceinstantiation is required for correct device identification and functionality(one device per address in this address range: 0x48..0x4f).

The DS1625 is pin compatible and functionally equivalent with the DS1621,but the DS1621 is meant to replace it. The DS1631, DS1721, and DS1731 arealso pin compatible with the DS1621 and provide multi-resolution support.

Additionally, the DS1721 data sheet says the temperature flags (THF and TLF)are used internally, however, these flags do get set and cleared as the actualtemperature crosses the min or max settings (which by default are set to 75and 80 degrees respectively).

Temperature Conversion¶

• DS1621 - 750ms (older devices may take up to 1000ms)
• DS1625 - 500ms
• DS1631 - 93ms..750ms for 9..12 bits resolution, respectively.
• DS1721 - 93ms..750ms for 9..12 bits resolution, respectively.
• DS1731 - 93ms..750ms for 9..12 bits resolution, respectively.

Note:On the DS1621, internal access to non-volatile registers may last for 10msor less (unverified on the other devices).

Temperature Accuracy¶

• DS1621: +/- 0.5 degree Celsius (from 0 to +70 degrees)
• DS1625: +/- 0.5 degree Celsius (from 0 to +70 degrees)
• DS1631: +/- 0.5 degree Celsius (from 0 to +70 degrees)
• DS1721: +/- 1.0 degree Celsius (from -10 to +85 degrees)
• DS1731: +/- 1.0 degree Celsius (from -10 to +85 degrees)

Temperature Resolution:¶

As mentioned above, the DS1631, DS1721, and DS1731 provide multi-resolutionsupport, which is achieved via the R0 and R1 config register bits, where:

R0..R1¶

The resolution mode for the DS1631, DS1721, or DS1731 can be changed fromuserspace, via the device ‘update_interval’ sysfs attribute. This attributewill normalize the range of input values to the device maximum resolutionvalues defined in the datasheet as follows:

The following examples show how the ‘update_interval’ attribute can beused to change the conversion time:

$ cat update_interval750$ cat temp1_input22062$$ echo 300 > update_interval$ cat update_interval375$ cat temp1_input22125$$ echo 150 > update_interval$ cat update_interval188$ cat temp1_input22250$$ echo 1 > update_interval$ cat update_interval94$ cat temp1_input22000$$ echo 1000 > update_interval$ cat update_interval750$ cat temp1_input22062$

As shown, the ds1621 driver automatically adjusts the ‘update_interval’user input, via a step function. Reading back the ‘update_interval’ valueafter a write operation provides the conversion time used by the device.

Mathematically, the resolution can be derived from the conversion timevia the following function:

g(x) = 0.5 * [minimum_conversion_time/x]

where:

• ‘x’ = the output from ‘update_interval’
• ‘g(x)’ = the resolution in degrees C per LSB.
• 93.75ms = minimum conversion time