question about MCLR PIN connection?!

Discussion in 'General Electronics Chat' started by Rawankhader, Nov 10, 2017.

  1. Rawankhader

    Thread Starter New Member

    Sep 22, 2017
    28
    1
    Hi All,,

    what is the difference between the following connections for MCLR PIN of PIC MCU ?


    tttttttttttttttttt.jpg
    why we dont connect The MCLR directly to VDD?

    what is the purpose of R,R1,C and D ?

    Regards
     
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    This looks and sounds like a homework assignment.
     
    Rawankhader likes this.
  3. Rawankhader

    Thread Starter New Member

    Sep 22, 2017
    28
    1
    No :)
     
  4. Raymond Genovese

    Active Member

    Mar 5, 2016
    957
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    MCLR=Master Clear pin External Reset.

    See here and consult the data sheet for whatever PIC you are using.

    In your first diagram, series resistor (R2) is used to prevent over-current damage from ESD voltage spikes. See here (figure 12.2 and related text), for example.

    Your second figure does not have MCLR connected to anything. If the pin is configured internally as reset rather than an input pin, it is pulled up, internally to Vdd. If your drawing should show a connection, it is the "older" style of pulling MCLR up.

    Your third figure shows the same scheme as your first figure plus a diode, to enhance ESD protection I think. It also shows a capacitor which will lengthen the MCLR pulse if the externally applied pulse is very short.
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    6,784
    The #3 drawing is for in-circuit serial programming and should include a switch to isolate (disconnect) the capacitor when programming and connect when the circuit is in use in an electrically noisy circuit (especially with inductive loads. The capacitor will insure the device does not reset unintentionally.

    The Device will not program if the capacitor is there. Or, it will program if Vpp is connected directly to/MCLR and R1 is at least 10k.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Picmicro use the the method shown on all their prototype boards, with the exception of the diode. The ICP Mclr pin is also connected to the MCLR pin.
    It is usefull if needing to reset during operation.
    Max.
     
  7. Raymond Genovese

    Active Member

    Mar 5, 2016
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    ICSP.jpg

    The top diagram is from Microchip. Is the diode positioned correctly in the TS's? I initially thought that circuit was from an ICSP application, and maybe I am just looking at it wrong but it did not seem right. Explain please?
     
  8. RichardO

    Senior Member

    May 4, 2013
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    It looks like the Microchip drawing is wrong. :eek:
    With the diode as it is shown, the resistor/capacitor on the diode's anode will not do anything useful. :(
     
  9. philba

    Active Member

    Aug 17, 2017
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    302
    I believe Vpp is high voltage (12, iirc). The diode is to prevent it from flowing on to the power rail.
     
  10. RichardO

    Senior Member

    May 4, 2013
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    That is true but the diode also prevents the reset circuit from driving the MCLR pin low.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    15,674
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    I have have had no problems using the PMicro boards with the circuit shown in #6, when using ICP.
    Max.
     
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