Current limiting resistor between ICs' port

Discussion in 'General Electronics Chat' started by sanmu13579, Apr 25, 2013.

  1. sanmu13579

    Thread Starter New Member

    Apr 12, 2013
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    0
    Hello,

    There are a MCU and a communication chip in my design, I want to use a MCU's GPIO port to control the Inhibit (INH) pin of that communication chip, do I need to add a Current limiting resistor between these two pins?

    The MCU's datasheet can be found at http://www.freescale.com/files/microcontrollers/doc/data_sheet/MC9S12XEP100RMV1.pdf

    The communication chip's datasheet can be found at http://www.infineon.com/dgdl/TLE625...c0de1&fileId=db3a30431936bc4b0119762569c71805

    Based on many existing design I know that there is no need to add that resistor, but I don't know the reason. On the page 1206 of MCU's datasheet, it states that the Instantaneous Maximum current for digital I/O pin is +/-25mA. And in the Table 3 (page 12) of the communication chip's datasheet, it only lists the max logic voltage applied to INH pin, but doesn't states the limit current that INH pin can endure, what if the current that flow into INH pin is more than 25mA which maybe destroy both the MCU output pin and chip's INH pin?

    Thanks
     
  2. TheComet

    Member

    Mar 11, 2013
    88
    12
    Look at page 10-11 in the communication chip's datasheet: It gives you an example of how to connect it to your micro controller.

    Just above that on page 6, you will also see an internal block diagram where you will notice a pull-up resistor on the input, and above that on page 5 you will see a list of pin descriptions, stating that the INH pin has an internal pull-up resistor of 20k. This isn't really that relevant, because your micro controller GPIO pin is probably a push-pull configuration (except during reset), but it's good to know that you don't need to use an external pull-up resistor.

    Getting to the point: No, you don't need to limit the current. Why would you even have to limit the current? There's nothing that would cause a high current flow, is there?

    The +25mA you read in the datasheet is the maximum current your micro controller is able to supply. This doesn't automatically mean that there WILL be 25mA flowing. Probably much less.

    A lot of people lately on this forum have been asking weird questions about limiting current when you obviously don't need to. I suggest going back a few pages and studying Ohm's law and Kirchoff again.

    http://forum.allaboutcircuits.com/showthread.php?t=69757

    TheComet
     
  3. sanmu13579

    Thread Starter New Member

    Apr 12, 2013
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    If the MCU output high, and the Mode Control module of the communication chip is high-impendence, the current I is very small (see the attached file). But what if the Mode Control module is not high-impendence? In the chip's datasheet I didn't find the statement which explicitly states that INH port is high impendence, it doesn't list the input impendence of INH pin?
     
  4. TheComet

    Member

    Mar 11, 2013
    88
    12
    You're right, it doesn't explicitly state it's high impedance.

    However, it does state it is an input pin. Generally you can just assume it's pretty high impedance.

    Also, lets say it's not high impedance. What use would the 20k pull-up resistor have then?

    TheComet
     
  5. sanmu13579

    Thread Starter New Member

    Apr 12, 2013
    4
    0
    Normally the on-state resistance of the MOSFET is very low, for example, 2 mΩ. So if I ignore the on-state resistance of the MOSFET in MCU, and use R_modecontrol to represent the input impedance of Mode Control module, the equivalent circuit will be like as the attached figure. Then the current flow into the Mode Control module will be VCC_MCU/R_modecontrol, if assuming that R_modecontrol = 100Ω, then the current will be 50mA. So I think the Mode Control module must be high impedance.
     
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