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???
09/21/05 13:49
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#101308 - Nothing is solved...
Responding to: ???'s previous message
Supriya said:
a series resistor with zener was 10k, reducing that to 4k7, solved whole issue

Nothing is solved! Jan already tried to explain, why your scheme will not work reliably. I try it again:
When you set P1.3 high, then output emits logic high level. In order to do this, standard quasi-bidirectional port driver topology turns-on an internal active pull-up. So, when you put a load to the port pin, in your case 4k7 resistor, then this internal active pull-up can source a current of up to 750µA. This will cause a voltage drop of up to 750µA x 4k7 = 3.53V, which is way too high to make the port pin assume logic low level!
The only remedy is to drastically reduce the load resistance seen to ground. Load resistance must be smaller than 2V / 750µA = 2k7, better, even lower than 0.9V / 750µA = 1k2, where 0.9V is the maximum input low voltage.

Your scheme, which seems to look like that



does also show an additional disadvantage:
Your 4k7 resistor forms a voltage divider in combination with Rx. In order to keep the level at output of comparator high, Rx must be kept very low, which heavily increases current consumption, especially if the 4k7 resistor needs to be decreased to less than 1k2, as demonstrated above!
A better approach is shown below:



Here the load resistance seen by P1.3 is adequately low (1k2). The 200R resistor is for protection purpose, if you eroneously set P1.3 low during experimentation.
This circuit, which only needs one transistor shows the disadvantage of increased current consumption. A better solution is to use two inverter stages for the level translation, as shown below.



Another option to accomplish this level translation is to use MC14504 chip, for instance.

Kai

List of 46 messages in thread
TopicAuthorDate
AT89c2051 port pin remains latched            01/01/70 00:00      
   1-to-0 transition current            01/01/70 00:00      
   OC comparator            01/01/70 00:00      
      a diode in series will do...            01/01/70 00:00      
      LM336-5            01/01/70 00:00      
         Price!! and speed?            01/01/70 00:00      
            Hmmmmmm...............            01/01/70 00:00      
               write PID sotfware for mcs51 micro.            01/01/70 00:00      
                  Start it as a new thread            01/01/70 00:00      
      Using LM339 only            01/01/70 00:00      
         LM339!            01/01/70 00:00      
            Pull up to 339            01/01/70 00:00      
               Why?            01/01/70 00:00      
               here are the two schemes            01/01/70 00:00      
                  not 12V, but 5.7            01/01/70 00:00      
                     and            01/01/70 00:00      
                        ???            01/01/70 00:00      
                           Question            01/01/70 00:00      
                              LM339            01/01/70 00:00      
                              open Collector            01/01/70 00:00      
                                 OOPS.            01/01/70 00:00      
                  12V swing            01/01/70 00:00      
                     there were three pictures...........            01/01/70 00:00      
                        yep...            01/01/70 00:00      
   Use P1.0 or P1.1            01/01/70 00:00      
      P1.0 and p1.1 are used            01/01/70 00:00      
   yep!! that is solved            01/01/70 00:00      
      Nothing is solved...            01/01/70 00:00      
         why so complicated?            01/01/70 00:00      
            Vil for 89c2051            01/01/70 00:00      
               then use a Schottkey            01/01/70 00:00      
                  LM339            01/01/70 00:00      
                     It probably will in the lab, the            01/01/70 00:00      
                        noise immunity?            01/01/70 00:00      
                           it most certainly does            01/01/70 00:00      
            Low-voltage noise margin            01/01/70 00:00      
               Real Programmers don't use Pascal            01/01/70 00:00      
                  It's so easy to state your opinion and t            01/01/70 00:00      
                     no more words            01/01/70 00:00      
                  It's amateurish to reduce noise margins            01/01/70 00:00      
                     hair-picking            01/01/70 00:00      
                        Fan-out specification            01/01/70 00:00      
                           Circuits that work on.....            01/01/70 00:00      
                     Rise times at 3MHz            01/01/70 00:00      
                        a small correction            01/01/70 00:00      
   I second the motion            01/01/70 00:00      

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