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???
02/26/08 13:48
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Msg Score: +1
 +1 Good Answer/Helpful		 #151485 - Calculating the maximum resistor
Responding to: ???'s previous message
Ap Charles said:
If you use a 5V system it requires 400uA Typ for the port to see logic 1-0 transition . Where as your 100K resistance is only providing 50uA . Now for 400uA to see you require 12500E resistance.


Designing to the typical specification means that half of the parts won't work in the system. You must always design to the worst case specifications, which are in this case 660uA at Vin = 2 volts. Assuming a Vol from the sensor of 0.4V, gives 1.6V/660 uA =2.4K maximum value.

I agree with Oliver that no resistor is needed in this application.


List of 40 messages in thread
TopicAuthorDate
P3.5 input problem            01/01/70 00:00      
   100k is way too large            01/01/70 00:00      
      Yup            01/01/70 00:00      
      ports1-3 vs. port0            01/01/70 00:00      
      a small correction            01/01/70 00:00      
   Page 4 datasheet says            01/01/70 00:00      
      Calculating the maximum resistor            01/01/70 00:00      
         Calculating the ...            01/01/70 00:00      
   80uA is not a maximum specification            01/01/70 00:00      
      re:resistor            01/01/70 00:00      
   Resistor            01/01/70 00:00      
      I would not            01/01/70 00:00      
         Resistor            01/01/70 00:00      
            Resistor ... again ...            01/01/70 00:00      
      basic troubleshooting first            01/01/70 00:00      
         the sensor and its application            01/01/70 00:00      
            Oh, max frequency is ~4kHz :-)            01/01/70 00:00      
         Please ignore the "basic troubleshooting"            01/01/70 00:00      
         pray tell why            01/01/70 00:00      
            Capacitance problems ...            01/01/70 00:00      
   You have totally misunderstood the situation            01/01/70 00:00      
      thank you Kai            01/01/70 00:00      
         maybe it is            01/01/70 00:00      
            Link in original post            01/01/70 00:00      
            datasheet            01/01/70 00:00      
         Being paranoid is a good attitude...            01/01/70 00:00      
            Paranoia medicine            01/01/70 00:00      
               Yes, perfect, i also thought about this...            01/01/70 00:00      
                  the "industrial strength" 485 transciever            01/01/70 00:00      
                     But they gaurantee this            01/01/70 00:00      
      But the micro port shouldn't be driving anything            01/01/70 00:00      
         Missing the boat.            01/01/70 00:00      
            Waving at the boat.....            01/01/70 00:00      
               wrong place (in some instances)            01/01/70 00:00      
            More about the P3.5 pull-up issue            01/01/70 00:00      
               one simple way            01/01/70 00:00      
               AT89S8252            01/01/70 00:00      
         Please read this...            01/01/70 00:00      
   ABS & Safety            01/01/70 00:00      
      safety concerns            01/01/70 00:00      

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