a problem in design counter circuit

[wael_moharam]

plz i need help , i made counter circuit use 4520 but there is a problem ,the circuit count random , why ? i dont know ,there is the first time i make counter circuit , i use 555 as clock because i need clock has various duty cycle .

 

[vfone]

Generally counters use a trigger circuit at the input, to eliminate the uncertainty between input noise and signal input. Is possible the threshold level of this trigger to be too low.

 

[wp100]

Hi,

If you post your circuit diagram and code then perhaps we can see what you have got before we can suggest what is causing the probelm.

 

[wael_moharam]

i notice that the instability in circuit is happend when i conncet power supply to circuit so the start point of count is random, although i use a capacitor bt ground and vcc . can i use 4538 to make delay until the circuit be stable (so i prevent it from count as soon as the power is applied ).i can do it by make anding gate bt 4538 &clock
or there is another solution . plz can you help me.

 

[bongkk]

You need a power up reset circuitry to the reset pins 7 and 15. This will ensure it starts from count zero each time you power it up. It also puts a delay before the counting starts. You could use a simple 10uF/ 100K combination.

 

[karesz]

Hi wael_moharam,

You MUST HAVE , generally, ALL INPUTS in "CMOS-World" at fixed potentials (on GND or VDD_ direct or with for ex. min. 50/100KOhm if you must overlist its level)_THIS IS VERY IMPORTANT, then you dont have controlled logic levels on their with time...
If it counts around 0`clock; I think RST can be fix at GND too.
CMOS circuits are very high Ohmic-Input systems/ICs = it has light some charge at a free Input (it will take some potential between 0 & Supply & it changes too...); you can long time search for reason of so a randomly variing situation...

Your CD/MM4520 is a good, but ca. 45 years old technik_for new designs you shuold take 74 ACxxx circuits or minimum 74AC/4520 (if it exist)...
Of course, if you have found it on your table_ than is clear.
K.

 

[karesz]

I did not find 74AC4520 yet, but this is another (good) logic selecting possibility:
https://www.onsemi.com/PowerSolutions/productList.do?id=8

Added after 1 hours 12 minutes:

I found it as 74HC4520_is functioning up to min. 30MHz with 5V...
K.