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CD4541B: CD4541B activated at initial state - Not expected output

Nerea Belategui
Prodigy 10 points
Part Number: CD4541B
Other Parts Discussed in Thread: SN74HC4060, CD74HC4060

We have implemented the component but we have some problems with it during our tests.

Configuration of the CD4541B :

Between pin 1 and 2: a Resistor and a capacitor R=56Kohm, C=100nF.
Pin 3: 120Kohm resistor connected to the middle of the C and R of pin 1/2.
Pin 4: NC
Pin 5: +3.3V
Pin 6: Connected to our Reset signal (output of a logic gate)
Pin 7: GND
Pin 8: Our output signal. Connected to the input of a logic gate
Pin 9: +3.3V
Pin 10: GND
Pin 11: NC
Pin 12: +3.3V
Pin 13: GND
Pin 14: +3.3V

At power supply increase (+3.3V) the output is activated during 1.6s. We do not understand why is activated because the MR pin is not activated. We have verified that there is no peak at this initial stage with the oscilloscope.

Could you please help us to understand why the output is activated at this initial stage of increase of the power supply?

  • 0
    Guru 297210 points

    The initial state after power on is undefined; see [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register) As long as MR  has not been activated, the output might be high or low.

    MR should be high during power on.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you for your answer.

    We use this device as a temperature alarm. If the MR is activated, the output is also activated for 14min (the output is connected to several devices' RST and consequently they will decrease their temperature). As you can understand, we cannot have devices that are switched-off during 14min each time after powering up. So how can we avoid this non-defined output? Can we put a pull-up / pull-down at the output? Do you recommend to change the device? Do you know the reference of another one that could do the job?

    Thanks in advance,

    Nerea

  • 0 in reply to Nerea Belategui
    TI__Guru* 96977 points

    Hi Nerea,

    My guess is that the system was designed with the assumption that any latched devices were starting in a known state (LOW is a common assumption, since many latched devices do start LOW more often than they do HIGH).

    Part of any system design should be to initialize any latched devices (counters, latches, flip-flops, shift registers, etc). It sounds like the system design would need to be updated - we don't have any direct replacements that include an internal initialization (that's what the MR pin is for).

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Hello Emrys and thank you for your answer! Do you mean that if we activate the pin MR during power-up we will have the same output state each time? So, the non-defined output will be now defined thanks to the activation of the MR?

    Thanks in advance.

  • +1 in reply to Nerea Belategui
    Guru 297210 points

    Yes; MR puts the device into a known state. But you have the problem that you are also using MR to trigger the temperature alarm state.

    The similar SN74HC4060 has the same problem.

    I think you need an external circuit that keeps the device in reset while idle. When a temperature alarm happens, MR is deactivated, and the timer starts. When the 14 min timeout has elapsed, the output signals makes the reset active again. This should be doable with a set/reset latch.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you so much. I have a last question, the datasheet says: ''A setup time equal to the width of the one-shot output is required immediately following initial power up, during which time the output will be high''. That means that in our case the MR needs to be activated during 14min? And that the output could no be useful during this time?

  • 0 in reply to Nerea Belategui
    Guru 297210 points

    Use the SN74HC4060/CD74HC4060 instead.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I do not understand well your answer. Just above, it was mentionned that SN74HC4060 had the same problem as CD4541B regarding the non defined output during power up. So why use this component instead of the original? Regarding the phrase quoted from the datasheet 'setup time [...] '. Do you know something about?

    Thanks in adavance.

  • 0 in reply to Nerea Belategui
    Guru 297210 points

    The SN74HC4060 works similar to the CD4541B (MR is CLR), but it does not do anything special at power up. You can simply keep CLR high during power up.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    'Does not do anything special' at power up? Does not have the output non defined at power up or does not need a setup time following initial power up?

  • 0 in reply to Nerea Belategui
    Guru 297210 points

    The state of the counter is undefined at power up (if not using CLR). It does not have that special setup time requirement.