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Power on reset question

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eTech

Well-Known Member
Hi everyone

I have a monostable 555 circuit that is used to generate an LED lamp test
signal. The signal is a single positive going 5v pulse of about 2 seconds and is generated at power up.

I need a POR signal to intialize a couple of 74HCT74 D flip flops (set Q to 0) and also clear a
74HCT163 chip (set all Q outputs to 0).

Can this same signal be used to reset the FF's and clear the 74HCT163 chip if I invert the output with an 74HCT14 inverter?

I'm thinking I can connect the FF RD pins and the 163 chip MR pin to the inverted signal.

Or do I need to provide a dedicated circuit?

thanks
 
Do you want the resets to occur on the rising edge or the trailing edge of the 555 pulse?
 
The chips require a hi to low transition to reset.
I'm planning to use an inverter at the output of the
555.
 
What does your 555 circuit look like? If you use the rising edge of the 555 the voltage may not be high enough for your logic gates to work properly yet and the output of the gate may just stay low (no edge). How about using the falling edge of the 555 2 second timer?
 
What does your 555 circuit look like? If you use the rising edge of the 555 the voltage may not be high enough for your logic gates to work properly yet and the output of the gate may just stay low (no edge). How about using the falling edge of the 555 2 second timer?

See attached

It works on a breadboard....
 
Hmm. I'm a little surprised, but you can't argue with success. ;) Have you tried it with a slow power up supply? The only reason I have concerns is that the 555 wakes up at a low voltage so it probably triggered from about .7 volts while the logic gate thershold is higher than that. The simulation will show the 555 timing, but I think the logic is instant on.
 
Hmm. I'm a little surprised, but you can't argue with success. ;) Have you tried it with a slow power up supply? The only reason I have concerns is that the 555 wakes up at a low voltage so it probably triggered from about .7 volts while the logic gate thershold is higher than that. The simulation will show the 555 timing, but I think the logic is instant on.

I'll give this a try.. :)

I think the reason this works is because at startup (power on)
the 555 has both pins 2 and 6 low, causing the output to be held low.
At the same time, the inverter completes power up and sees a
low on its input and outputs a high.
At this point, the FFs and shift reg have finished power on and see
A high at their POR pins...

Whew...

Ok....(deep intake of air)

Shortly thereafter...the 555 completes startup and begins it one-shot
cycle and outputs a high...causing the inverters output to transition from
high to low...this transition is then seen by the FFs and shift reg POR pins
as a signal to reset...so they perform a reset.

eT
 
Yes, that was my concern, that the 555 starts faster than the inverter. I can't model power on for the inverter, but this shows what I was concerned about.
 
Yea..I can see where the 555 completing power up before the inverter might
prevent a reset. But I think the chances of that are slim. The 555 has
to wait for the voltages on pins 2 and 6 to stabilize before it will trigger.
By that time the inverter has already completed power up.

eT
 
The HCT parts do not need a high to low transition for reset. All they need is reset pin held low after power is applied. A simple RC power on reset circuit should work fine. Just ensure reset remains low until power is stable, adjust RC network for timing. See below.

POWER ___/
RESET ______/

View attachment 65752
 
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