AVR-Chat

On Sun, 04 Dec 2005 14:48:19 -0000 you wrote:

As you noted the ADC can be run with a faster clock. If you only need a few bits
resolution then I don't see why there should be a problem. 200KHz gives 10 bits.

>I have an ATmega128 in an existing circuit that reads the value of a
>resistor in an attached cable to determine the identity of an attached
>device.  This has worked well, but they are out of resistor values
>that they can easily distinguish.  I am considering the Dallas OneWire
>devices as a possible work around.  
>
>There are nearly 100,000 of these units in the field and the new
>product replacing the old one has the same circuit.  This circuit is
>the problem.  The ADC input can be configured as a digital IO, but
>there are two RC filters between the ADC and the cable attachment.  In
>between the two RC filters is an open drain transistor pull down and a
>5.6K pull up.  The RC between the pull down and the cable is RC=1 uS.
> The RC between the pull down and the ADC input is 22 uS.  
>
>I can live with the short RC by making my minimum pull down time 3 uS.
> But the long RC will prevent me from using the digital input to see
>the response from the slave on a read.  The minimum hold down time for
>the slave is only 15 uS.  At that time the voltage at the ADC will be
>about half the pull up voltage.  
>
>So to make this work I will need to use the ADC.  The spec timing is a
>200 KHz clock (faster with poorer resolution).  It seems to take 1
>clock to synchronize and 1.5 clocks to do the S/H, another 12 clocks
>for a result.  At 200 KHz, 2.5 clocks is almost the perfect sampling
>point of 15 uS, but I have no idea of the size of the aperature window.  
>
>Anyone have experience with precise timing of the ADC like this?  I
>need to distinguish between 50% of Vcc and 95% of Vcc, so the
>resolution requirements are not high.  Even 6 bits should give me
>plenty of head room.  But the timing is critical.  At 15 uS the
>waveform will start ramping back up so I need to sample as close to
>that time as possible.  
>
>Is this practial or even feasible?  My local disti FAE is not very
>familiar with the AVR so I can't ask for help from him.  Will Dallas
>support be of any use on this or do I need to test it myself? 
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>Yahoo! Groups Links
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--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net