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chronic
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Posted: 17 April 2018 at 11:01pm | IP Logged Quote chronic

Now I see why you cut the ground connection, the 31855 doc says
expressly to not connect the thermocouple to ground, the 6675 schematic
and pcb has it deliberately connected !
Weird - looks like they stuck the 31855 ic on a pcb designed for
the 6675. Commiserations Mik.

Edited by chronic on 17 April 2018 at 11:02pm



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bigmik
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Posted: 18 April 2018 at 6:56am | IP Logged Quote bigmik

Hi All,

I played a little more before we had to go out to watch “Priscilla Queen of the Desert” (Great show by the way, WE had front row seats)..

If I change the SPI mode from mode 2 to mode 0,1 or 3, the readings (at room temperature only tested) were within 1.5degrees..

I used a cheapa$$ electronic thermometer that showed my room was 19.3C (making the original code 9C low.. way too far out..) with either of the other 3 modes it returned 21 +- 0.5C.


I will need to throw my head into the spec sheet to work out which mode should be best for the MAX6675.

Regards,

Mick

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BrantB
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Posted: 11 May 2018 at 2:12am | IP Logged Quote BrantB

I know I am coming late to the game here, after several veterans have already weighed in ... but, if you're like me, these types of unresolved issues will haunt you. (At least, I know I have that character flaw; I envy you if you don't. ) So, for what it is worth, here is what I would do (after a few days away from it, so I was looking at it all with fresh eyes) ...

I see from the datasheet that CS can be taken high at any point (i.e., it is not necessary to clock the entire 32 bit stream). I also see that, according to the datasheet, the first bit is presented to the SO (MISO) pin based on the falling edge of the CS (not necessarily correlated to the clock signal, if I read that correctly). I don't recall many (any?) other SPI streams being described this way, so part of me thinks that this may be significant. Having said that, I must admit I have only ever used SPI chips which have MISO and MOSI (as opposed to something like this one, which is most definitely output-only).

Armed with those observations and the results that you have so far been getting, I would be very tempted to write my own bit-banging routines so that I could control every aspect of every step of the SPI process, and examine all of the pertinent results (at each individual step) with PRINT statements. I have resorted (successfully) to this sort of technique before - but in another environment (using the BASCOM compiler with Atmel's AVRs and/or Xmegas).

And, I would only bother looking at the first 14 bits, since they are all that would be required to gain sufficient insight, and ultimately show success.

I don't have one of the MAX31855 chips/boards, otherwise I would happily whip up some routines and dig into it a "bit" myself (I obviously could not resist the bad pun ). This one has me really curious.

Sorry if that doesn't end up helping much, but I wanted to at least try to contribute something to towards your trials and tribulations. We've all been there at some point (probably more than once), and sometimes the eureka moment comes from left field ... so, consider this a wild pitch.

Good Luck!

Brant
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bigmik
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Posted: 11 May 2018 at 12:39pm | IP Logged Quote bigmik

GDay Brant,

I suspect that you may be right there.

I have got the MAX6675 working and doing what I needed (but my project has stalled due to time constraints) so I have shifted to that chip.

If you PM me your address I am more than happy to send you a PCB and thermocouple to play with.. I have 3 here and they all do the same..

I cannot believe that all 3 boards are faulty (I may have killed one by plugging 5V in instead of 3v3).

Let me know if interested.. They are of no use to me with no working software.

Regards,

Mick

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BrantB
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Posted: 11 May 2018 at 10:39pm | IP Logged Quote BrantB

Hi Mick,

Time constraints ... boy, I know exactly what you mean.

In fact, funny you should mention that. I am quite happy to take a crack at this if you send me a board, provided you're not in a rush. It will likely be a little while before I can have a look at it, but I will work on it.

I'll also post back my findings (along with working code, hopefully). Although (still) a newbie to the forum and to the MM family, I have a great deal of respect for the high functioning of this forum. Members are respectful of, and helpful to, each other, and everyone seems to work towards the greater good. I will be happy to make a contribution to the ecosystem.

I'll PM you with my address. And no need to panic on my account.

Cheers!

Brant
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