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Re: Identification of modules

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On 3/31/2017 6:39 AM, John Devereux wrote:
> rickman <gnuarm@gmail.com> writes:
>
>> On 3/31/2017 3:21 AM, Piotr Wyderski wrote:
>>> My system will be composed of a number of free slots
>>> which can be populated with some modules (mostly
>>> containing power electronics). The simplest the module
>>> is, the better, don't want to use a micro to do the
>>> job because of its limited FLASH/EEPROM retention time.
>>>
>>> The module must be able to report its identity to the
>>> main controller (one from the closed universe of, say,
>>> 32 possibilities known in advance) and which slot it
>>> occupies. A very useful feature would be to have
>>> a temperature sensor of some kind for moderate overload
>>> detection.
>>>
>>> I cannot afford many pins to do that, so the simplest
>>> 5+5 pin solution of binary encoding the relevant
>>> information is not an option.
>>>
>>> I came up with the idea to (ab)use the Maxim's DS2438
>>> battery monitor and use its voltage/current sensor A/D
>>> channels to fetch the value encoded by a resistive divider.
>>> The temperature sensor is there for free and the cost is
>>> below a buck for 20 pieces in China. It supports the 1-wire
>>> bus.
>>>
>>> Do you know anything simpler/better or is there
>>> anything to worry about that Maxim part?
>>>
>>> BTW, why are the Maxim parts so insanely expensive?
>>> I could find many applications for the 1-wire DS2450
>>> A/D converter, but the cost makes it pointless -- one
>>> can emulate it with any SO8 micro for 1/4 of its price.
>>
>> I'm not clear on what you are using the resistive divider for.  Is
>> that to identify the module type or the slot?
>>
>> The lowest priced 1 wire part I found on Mouser is the DS28E05P+T.  It
>> combines the unique 64 bit serial number each 1 wire device has with
>> 112 bytes of user EEPROM for under $0.50 in quantity.  The EEPROM can
>> be programmed in manufacturing to provide a module type identification
>> number.
>
> We use those (with trepidation, maxim etc). We use it to store the
> serial number and calibration data on a tiny sensor module, to make them
> interchangeable.

That is pretty much exactly what they are intended for.  I see now that 
part is barking up the wrong tree.  He doesn't care about the serial 
number or the flash, he only cares about the ADC.


>> I'm not clear on what your limitations are for identifying the slot
>> number.  The easy way is to isolate the 1 wire buses to each slot so
>> you are only talking to one at a time.  If you don't want to use that
>> many pins on your MCU, an analog switch multiplexer will allow the MCU
>> to talk to one slot at a time and only need log2(N) I/Os.
>
> You don't need to do that AIUI, the one-wire bus has a discovery
> algorithm you can use to address any device when all the "one-wires" are
> connected together. Never had to use it but it's there.

I've used it before, how else do you learn what devices are on your bus? 
   Addressing the devices is not the same as knowing what slot they are 
in.  That is the purpose of the multiplexers, to isolate the ID pin on 
each card slot so the slot is known.  Instead he wants to supply to the 
slot an analog voltage through a resistive divider which is read by an 
ADC on the card.  Then he doesn't like the cost of the ADC.

There are other ways to get an ADC such as an MCU, but I can't think of 
any that won't use flash memory... except for the TI MSP430FR devices 
that use ferroelectric storage, FRAM.  But I doubt that has enough 
retention time for this requirement either.

The only other ADC that might suit him is one that works over SMBUS. 
That will use two pins, but it is all a bus.  The ADC needs two inputs, 
one for the slot voltage and one for the card type voltage.  Best would 
be one that can do ratiometric measurements to eliminate the effect of 
variations in Vcc.

-- 

Rick C

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Thread

Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 09:21 +0200
  Re: Identification of modules Don Y <blockedofcourse@foo.invalid> - 2017-03-31 01:31 -0700
    Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 10:36 +0200
      Re: Identification of modules Don Y <blockedofcourse@foo.invalid> - 2017-03-31 02:16 -0700
        Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 11:39 +0200
          Re: Identification of modules Tom Gardner <spamjunk@blueyonder.co.uk> - 2017-03-31 11:32 +0100
            Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 12:59 +0200
          Re: Identification of modules Don Y <blockedofcourse@foo.invalid> - 2017-03-31 11:55 -0700
    Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 11:30 +0200
  Re: Identification of modules rickman <gnuarm@gmail.com> - 2017-03-31 05:20 -0400
    Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 11:53 +0200
      Re: Identification of modules rickman <gnuarm@gmail.com> - 2017-03-31 06:16 -0400
        Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 12:39 +0200
          Re: Identification of modules rickman <gnuarm@gmail.com> - 2017-03-31 13:27 -0400
            Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 22:14 +0200
              Re: Identification of modules rickman <gnuarm@gmail.com> - 2017-03-31 17:48 -0400
                Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-04-03 10:57 +0200
    Re: Identification of modules John Devereux <john@devereux.me.uk> - 2017-03-31 11:39 +0100
      Re: Identification of modules rickman <gnuarm@gmail.com> - 2017-03-31 13:38 -0400
        Re: Identification of modules Piotr Wyderski <peter.pan@neverland.mil> - 2017-03-31 22:21 +0200
  Re: Identification of modules Joe Chisolm <jchisolm6@earthlink.net> - 2017-03-31 13:02 -0500
  Re: Identification of modules Tim Wescott <seemywebsite@myfooter.really> - 2017-03-31 16:18 -0500

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