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Fuse bits?

Discussion in 'Microcontrollers, Programming and IoT' started by Maisha, Apr 25, 2016.

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  1. Maisha


    Apr 22, 2016
    Can anybody pls explain what is a fuse bit and what it does?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Fuse bits are 1 bit controls in a programmmable component that are activated or de-actovated (fused) once when programming the device to define a certain behavior.

    Fuse bits come in many flavors.

    An example are fuse bits thst define the source and the frequency range for an on-cip oscillator of a microcontroller.
    A microcontroller often has more than one possible clock source, e.g. an RC-oscillator, a clock quartz or a high frequency quartz oscillator. In order to know which clock source is used in a certain circuit, fuse bits are set to tell the microcontroller which clock to use. Without this information the controller would not be able to start upo at all.

    Other fuse bits are used for different purposes. The datasheet of the programmable component will give you all details.
  3. Maisha


    Apr 22, 2016
    thank you:)
  4. bob weir

    bob weir

    Sep 10, 2014
    if you follow this link and find a 3-18-16 posting you will see a code example.
    the image has 47 lines of assembly code for the 12F508 uC.
    and line 3 shows how its 4 fuse-bits are configured.

    this makes the GP3/MCLR pin on the uC function as a reset input.
    this is handy when you want to reset the uC without having to cycle the +5V supply

    if instead it reads
    _CONFIG _MCLRE_OFF .....
    then this same pin is now available to be used as a digital input pin. and if you want to
    reset the uC now you will have to cycle the 5V power.

    to learn the other fuse-bits that can be set for a uC chip download its documentation.
    and find the section 'configuration bits' . for this 12F508 that'd be on p. 41 of
    the '08 version of the doc' = DS41236E

    this 508 uC has 4 bits that are configurable. yet only one has a 'fuse' nature.
    that'd be the CP bit. code-protect. by fuse-nature i mean once its 'ON' the code
    loaded onto the uC cannot be changed/edited any further. so be careful with
    that bit. or it'll cost you the price of a new chip if you want to edit the code any

    on line 3 look for ....... & _CP_OFF & ....

    the other 3 fuse-bits for the '508 can be changed as many times as you want to on
    the same uC chip.

    i dont know why they are called 'fuse bits' . guess its a leftover term from the days
    of earlier uC designs
    Last edited: May 1, 2016
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    These used to be real 'fuses': contacts that could be opened by forcing a high current through them, the same principle as triggering as fuse. This meant these bits could be 'programmed' only once and only on a programmer capable of delivering the required high voltage. Thus the settings controlled by the fuse bits were protected from manipulation by software running amok.
    Today fuse bits are most often realized by FLASH based mechanisms so they can be re-programmed. Bit not by the software in normal operation, only during a dedicated programming cycle.
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