I just realized Paul's explanation below didn't get posted to the forum,
and thought it would be of interest.
I also wanted to correct something I stated earlier. As currently
implemented owfs does not support the MAX31850/851. Though owfs will
properly display low temperature values for these devices, at higher
temperatures I believe the high order bits will be treated as sign bits (I
don't know the details of the code involved). This would result in higher
temperatures being displayed as negative and truncated values.
The reason for this is that the high order bits are handled as extended
sign bits in the other devices of this family.
Paul Panish
---------- Forwarded message ----------
From: *Paul Alfille* <[email protected]>
Date: Thursday, February 6, 2014
Subject: [Owfs-developers] Planned Device Support
To: Paul W Panish <[email protected]>
Paul Panish's question is how to distinguish the DS1825 MAX31826 MAX31850
and MAX31851.
DS1825 is a temperature sensor with temperature limits
http://datasheets.maximintegrated.com/en/ds/DS1825.pdf
MAX31826 is a temperature sensor with 1K eeprom
http://datasheets.maximintegrated.com/en/ds/MAX31826.pdf
MAX31850 and MAX31851 are thermocouple sensors with internal (cold
junction) temperature
http://datasheets.maximintegrated.com/en/ds/MAX31850-MAX31851.pdf
All share the same family code (0x3B) and if there are specific ranges of
ID for them, Maxim isn't saying.
This is my current strategy for distinguishing them:
1. DS1825 has bit 7 of configuration byte = 0 All MAX devices have bit7=1
2. Bytes 2&3 (0 indexed) of the scratchpad is 0xFF 0xFF for the MAX31826
it isn't for the MAX31285X, even at startup.
3. I don't know of any way to tell which thermocouple is connected, but the
chip should be matched properly and hides that detail.
4. I don't know how to distinguish the MAX31850 and MAX31851, but it's not
clear that it is necessary.
On Thu, Feb 6, 2014 at 12:46 PM, Paul W Panish
<[email protected]<javascript:_e(%7B%7D,'cvml','[email protected]');>
> wrote:
> Paul,
>
> One other question, how will you determine what type of device is in use
> since the family code doesn't give you that information? It seems you'll
> need to know the actual device type to properly interpret the data, as well
> as to present the directory structure elements for each device.
>
> Can you non-destructively probe for information from which you can infer
> the type, or is there something else I'm missing?
>
> Paul P.
>
>
> On 02/06/2014 12:00 PM, Paul W Panish wrote:
>
> Hi Paul,
>
> You definitely want the default to be the cold junction compensated
> temperatures in bytes 0 and 1. I guess you could also provide the cold
> junction temperature in bytes 2 and 3, which would provide ambient
> readings, but I don't think that's really necessary.
>
> I hadn't looked too closely at the differences between the MAX31826,
> DS1825 and the MAX31850 in terms of owfs support. I guess the reason I got
> valid temperatures was that I checked at room temp, so the sign bits didn't
> matter. I assume if I'd actually measured high temperatures the msb values
> in byte 1 would have been ignored and a low temperature would have been
> shown regardless.
>
> Paul P.
>
> On 02/06/2014 10:29 AM, paul.alfille wrote:
> > The chip gives 2 temperatures, and some error flags.
> >
> > Which temperature should be tge default temperature? I think the
> thermocouple one sincere that's the real point of the chip.
> >
> > I guess there will be a subdirectory for the other features. All
> temperatures will have same resolution since that's what the chip gives.
> >
> > Any suggestions or alternatives?
> >
> >
> > Sent via the Samsung Galaxy S(tm) III, an AT&T 4G LTE smartphone
> >
> >
> >
> > -------- Original message --------
> > From: Paul W Panish
> > <[email protected]><javascript:_e(%7B%7D,'cvml','[email protected]');>
> > Date: 02/06/2014 8:12 AM (GMT-07:00)
> > To: Paul Alfille
> > <[email protected]><javascript:_e(%7B%7D,'cvml','[email protected]');>
> > Subject: Re: [Owfs-developers] Planned Device Support
> >
> >
> >
>
>
>
>
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