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'[PIC:] shift register(s) to read temp from 35 DS18'
2004\09\06@175835 by mmynsted_news

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I am new to this, and a hobbyist, not an EE, so would anybody recommend a shift register compatible with a PIC 16F628 based design that could be used to allow me to read temp from about 35 DS1820 sensors on as few I/O pins as possible?  The idea is I know that I will need to deal with more I/O than I want to have built-in to my PIC.  Reliability, and low power consumption are far more important than speed, if that makes any difference.  Has anybody done something like this?  (I do not want to use a handful of small shift registers, I would rather find one larger chip.)

If I look at a site like http://www.mouser.com and search for shift register I get many results.  I suspect I would want to limit by DIP, and 8 bit too, but that still leaves many options.
--
MM

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2004\09\06@182319 by Marc Nicholas

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Hi...

You've totally lost me here....you only need 2-wires (GND + signal) to
read all 35 sensors -- that's the idea of the 1-wire bus that Dallas
developed; it's a mini network.

Or am I missing something here?

-marc

{Original Message removed}

2004\09\06@184111 by Byron A Jeff

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On Mon, Sep 06, 2004 at 04:58:34PM -0500, mmynsted_news wrote:
> I am new to this, and a hobbyist, not an EE, so would anybody recommend a
> shift register compatible with a PIC 16F628 based design that could be used
> to allow me to read temp from about 35 DS1820 sensors on as few I/O pins as
> possible? ?The idea is I know that I will need to deal with more I/O than I
> want to have built-in to my PIC. ?Reliability, and low power consumption are
> far more important than speed, if that makes any difference. ?Has anybody
> done something like this? ?(I do not want to use a handful of small shift
> registers, I would rather find one larger chip.)

Um. This is a quote directly from DallasSemi's own page on the DS1820
----------------- Quote ----------------------
Each DS18S20 has a unique 64-bit serial code, which allows multiple DS18S20s
to function on the same 1-Wire bus; thus, it is simple to use one
microprocessor to control many DS18S20s distributed over a large area.
----------------- End Quote ----------------------

So why would you need more than a single I/O line?

BAJ
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2004\09\06@184955 by mmynsted_news

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LOL!  Thank you.

OK.  Originally I was going to use LM34s (maybe they may be networked too, I will check.).   The same idea is still of interest to me though, because I know that I will encounter this same problem. (I will check http://www.maxim-ic.com/ for their other sensors too, this 1-wire design is obviously really nice.


--
MM

On Monday 06 September 2004 5:20 pm, Marc Nicholas wrote:
> Hi...
>
> You've totally lost me here....you only need 2-wires (GND + signal) to
> read all 35 sensors -- that's the idea of the 1-wire bus that Dallas
> developed; it's a mini network.
>
> Or am I missing something here?
>
> -marc
>
> {Original Message removed}

2004\09\06@185610 by Alexandre Guimaraes

face picon face
Hi,

>You've totally lost me here....you only need 2-wires (GND + signal) to
>read all 35 sensors -- that's the idea of the 1-wire bus that Dallas
>developed; it's a mini network.
>Or am I missing something here?

   Take a good look at the datasheet... You need much more power than the
pull-up can supply to measure the temperature, specially with 35 sensors.
You need to use a active pull-up system or put one more wire to carry
power.. In my opnion I prefer to just put one more wire and be free from the
trouble...

Best regards,
Alexandre Guimaraes




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2004\09\06@191533 by Marc Nicholas

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Alexandre...

You don't have to issue a convert command to all 35 sensors at once!

-marc

-----Original Message-----
From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu] On Behalf
Of Alexandre Guimaraes
Sent: Monday, September 06, 2004 6:57 PM
To: Microcontroller discussion list - Public.
Subject: Re: [PIC:] shift register(s) to read temp from 35 DS1820
basedsensors

Hi,

>You've totally lost me here....you only need 2-wires (GND + signal) to
>read all 35 sensors -- that's the idea of the 1-wire bus that Dallas
>developed; it's a mini network.
>Or am I missing something here?

   Take a good look at the datasheet... You need much more power than
the
pull-up can supply to measure the temperature, specially with 35
sensors.
You need to use a active pull-up system or put one more wire to carry
power.. In my opnion I prefer to just put one more wire and be free from
the
trouble...

Best regards,
Alexandre Guimaraes




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2004\09\06@192844 by Marc Nicholas

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No worries.

As someone else alluded to, be careful with current
requirements....while you'll have no trouble "seeing" all 35 devices on
the 1-wire network, issuing a temperature convert command to all 35 at
once will get you into trouble. Assuming that you don't need terribly
speedy response time, you can just have your code cycle through each
sensor.

Or, you can beef up power on the 1-wire network with an additional +5vdc
line, or with a capacitor and Schottky diodes.

-marc

{Original Message removed}

2004\09\06@193931 by Kevin

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face

Try this for a working example and some pointers.
http://www.phanderson.com/tm125.html

Kevin

On Mon, 6 Sep 2004, mmynsted_news wrote:

{Quote hidden}

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2004\09\07@141157 by hilip Stortz

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you should also look at I2C and SPI, both 2 wire interfaces which are
sometimes useful (i.e. the timing isn't critical as it is on 1-wire
parts.).  note however that if the timing is a problem for you, maxim
does make a serial to 1-wire converter chip which takes care of the
critical timing for you (though i think you still have to keep up with
the serial port, you don't have to generate and read pulses of just the
right length).

mmynsted_news wrote:
>
> LOL!  Thank you.
>
> OK.  Originally I was going to use LM34s (maybe they may be networked too, I
> will check.).   The same idea is still of interest to me though, because I
> know that I will encounter this same problem. (I will check
> http://www.maxim-ic.com/ for their other sensors too, this 1-wire design is
> obviously really nice.
-----

-- Philip Stortz--"In Germany they came first for the Communists, and I
didn't speak up because I wasn't a Communist. Then they came for the Jews, and I didn't speak up because I wasn't a
Jew. Then they came for the trade unionists, and I didn't speak up because I
wasn't a trade unionist. Then they came for the Catholics, and I didn't speak up because I was a
Protestant. Then they came for me, and by that time no one was left to speak up."
-- Martin Niemöller, 1892-1984 (German Lutheran Pastor), on the Nazi
Holocaust, Congressional Record 14th October 1968 p31636.

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2004\09\07@144322 by Dave VanHorn

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At 01:13 PM 9/7/2004, Philip Stortz wrote:

>you should also look at I2C and SPI, both 2 wire interfaces which are
>sometimes useful (i.e. the timing isn't critical as it is on 1-wire
>parts.).  note however that if the timing is a problem for you, maxim
>does make a serial to 1-wire converter chip which takes care of the
>critical timing for you (though i think you still have to keep up with
>the serial port, you don't have to generate and read pulses of just the
>right length).

Personally, I find all the short time delays very annoying.
Too short to do it with a timer, and release the task.
Too long to avoid "sit and spin" loops.

Clocked serial is much nicer.

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