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SN74HC14: About the output state of SN74HC14 during power up

Part Number: SN74HC14

1 if give a rc curcuit in the input of 74hc14,the output state is a certain high during power up?

2 if the input of 74hc14 is pull down,the output state is a certain high during power up?

the curcuit is in the following filecurcuit.docx

  • Hello, and welcome to the forums!

    The SN74HC14 is guaranteed to operate between 2V and 6V. From 0V to 2V, the output state is unknown, but it will likely start to operate normally somewhere between 1V and 1.5V.

    The time constant of the RC for the top circuit is 115.1k * 0.47u = 54.1ms. The input is a valid 'low' for about half of one time constant, so around 27ms.  This assumes that the supply comes on very quickly (under 1ms). A slow supply transition can negate this.

    The bottom circuit won't do anything -- the capacitor is never changed, so the RC doesn't have any effect.

  • thanks for your reply,i still have some question

    1 why the "valid"low is about half of one time constant?

    2 why the supply need to comes on very quickly (under 1ms)?in my cuicurt,the supply need to come on about 15ms

  • Hello,

    (1) Is just because of how the math works out.  An RC causes a charge as described by this equation:

    V(t) = VCC * ( 1 - exp( -t / (R*C) ) )

    If we set V(t) to 0.4*VCC (about as high as you can get for a valid logic low input), then solve for t, we get:

    t = RC * (- ln| 0.6 | )

    and - ln| 0.6 | = 0.51

    So, that means the input is less than 0.4*VCC for the first 0.51 time constants.

    ---

    (2) I over-estimated quite a bit here -- 1ms is much faster than you need. Your supply turn on at 15ms will be fine.  Here's a sim to show the effect of the slower transition rate:

    Both of the above rise times (1ms to 5V and 15ms to 5V) are good. You can see that my cursor shows the turn-on voltage of the device (2V supply) and the input is still well within the V_IL range to be a valid logic 'low.'

    I simulated again with a much slower turn-on time of 200ms, and here you can see the effect I was referring to:

    Note that the cursor is again at 2V, but now the RC is charged to nearly 1V already, which may not register as a logic low.