74HC192 preset to 1, but powers on at 9

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dsegel

Joined Jul 10, 2010
1
This is part of a 12-hour clock circuit I'm designing just to teach myself more about logic. The problem is that the 74192 powers on into a count of 9 instead of 0; after that it counts to 10, 11, 12, then back to 1, and counts properly 1-12-1 from then on. Is there a way to fix this so that it starts at 0 initially? Why is it starting at 9?
 

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AlbertHall

Joined Jun 4, 2014
11,382
If you want it to be set to 0 at power up then hold the reset pin at 0 for a short period at power up. You could use a circuit like the one below.
 

RichardO

Joined May 4, 2013
2,271
Your circuit shows that the CLR input on the 74HC192 is not connected. All inputs of 74HC parts must be connected to a logic level.

The circuit given by @AlbertHall will do this.
 

dl324

Joined Mar 30, 2015
12,911
teach myself more about logic.
Since you're trying to learn, you should study the input and output voltage requirements for 74LS, 74S, and 74HC.

The minimum guaranteed HIGH level output voltage from 74S isn't high enough to satisfy the minimum HIGH level input voltage for 74HC.
 

MrChips

Joined Oct 2, 2009
23,851
Flop-flops will power up to any random state. It is the circuit designer's responsibility to implement a power-on reset (POR) if deemed to be necessary.
 
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