divide 3 counter

8/3/2019 Divide 3 Counter

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AND8001/DOdd Number Divide ByCounters With 50% Outputsand Synchronous ClocksPrepared by: Cleon Petty and Paul ShockmanProduct ApplicationsON Semiconductor

The application inquiries handled by the ProductApplications gives opportunities to solve customer needswith new ideas and learn of ways the customer has used ourdevices in new applications. A couple of these calls lead totechniques of designing odd number counters withsynchronous clocks and 50% outputs.The first technique requires a differential clock, that has

a 50% duty cycle, a extra Flip Flop, and a gate to allow Oddintegers, such as 3, 5, 7, 9, to have 50% duty cycle outputsand a synchronous clock. The frequency of operations islimited byTpd of the driving FF, Setup, and Hold of the extraFF, and the times cannot exceed one half on the incomingclock cycle time.The design begins with producing a odd number counter

(Divide By 3 for this discussion) by any means one wishes

0 6 ,5 em lc on dtl'CW rjttvm~iy 01 1 D . i , , 1 ' s t o n of Ud.l~

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APPLICATION NOTEand add a flip flop, and a couple of gates to produce thedesired function. Karnaugh maps usually produce countersthat are lockup immune.Example:Specify, Divide By 3,

50% duty cycle on the outputSynchronous clocking50% duty cycle clock in

Using D type Flop flips and karnaugh maps we find;Ad =A*B* and Bd =A

(Note: * indicates BAR function)Figure 1 shows schematic and timing of such a design.

ho Q D QA B

Q - Q -c cI I

Divide By 3

Figure 1.

Semiconductor Components Industries, LLC, 1999October, 1999 - Rev. 0 Publication Order Number:AND80011D
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AND8001/DUsing the technique, we add a gate on the clock to get

differential Clock and Clock bar, a flip flop that triggers onthe Clock Bar rising edge (Clock Neg.) to shift the output of

"B" by 90 degrees and a gate to AND/OR two FF output toproduce the 50% output. We get Figure 2, a Divide By 3 thatclocks synchronously with 50% output duty cycle.

~D I ~D Q D QA B C

Q- Q r-- QC C CI I I

50% Out

Clk InDivide By 3 W/50% out

ClkAQBQ

LUTFigure 2.

The Max frequency of the configuration (figure 2) iscalculated as Clock input freq'/2 =Tpd of FF "B" + Setupof "C" + Hold of "C".

Example:ADivide By 3 design has all possible states shown in chart1 but uses only the states shown in chart 2 leaving the states2,3,4,5, & 7 for possible lockup.Example:

Tpd = INs, Setup = !NS and Hold time =ONs.with these numbers the Max Frequency the configuration

can expect is; Cycle time =2* (1 + I)Ns or 4 Ns that convertsto 250MHZ.The Method is usable on other divide by "N" counters as

well by using the same methodology. The use of differenttypes of Flip Flops (J,K, S,R, Toggle, ETC.) may producefewer components. The type logic used may also dictateconfiguration. The configuration should always be checkedfor lockup conditions before the design is committed to aproduction.

Chart 1 Chart 2A B C A B C

0 0 0 0 0 0 0 01 1 0 0 1 1 0 02 0 1 0 6 0 1 13 1 1 04 0 0 15 1 0 16 0 1 17 1 1 1

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AND8001/DWe need to know that the counter will go into the flow,

shown in chart 2, if it happens to come up in one of theunused states at powerup or for any other reason. Figure 3shows the resulting flow chart of the analysis of the Divide

By 3 counter of Figure 2. There is no state that the countercan begin in that doesn't lead to the desired flow after oneclock cycle.

Observation shows that FF "C" follows FF "B" by a halfa clock cycle and will never be able to lockup making theanalysis of the Divide By 3 sufficient to assure the wholeconfiguration will have no lockup flow. So; only the 11 stateof the divide by three needed to be confirmed.The method is extendible to other odd larger divide by "N"

numbers by following the same design flow.a) Design a stable UP or Down divide by "N" counterb) Make the Clock input a 50% duty cycle differentialsignalc) Add a FF to follow one of the FF's in the counter by112clock cycled) OR!AND the shifted FF with the one that is driving itto obtain the desired 50% output

Figure 3.

Example:Design a 50% Divide By 9Use "D" type FF's, other types may give smaller

component countKarnaugh maps yield:Ad = A*B* Bd = A*B + AB*Cd=ABC*+CB*+A*C Dd=ABC

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AND8001/D

lli~D

.---

~D Q~~Q D Q D Q D~ OulA B C D EQ Q Q Q QC C C C C

Clk 1 IC 1 ICDivide By 9 50% Counter

ClkAQ

DQEQ

OUTFigure 4.

Choosing touse "C" as the flip flop to delay by a 112clockcycle is necessary to accomplish the 50% output requiredwhen" ANDed" with "E".

Another Synchronous 50% counter for Divide By 6, 10,12, 14, 18, etc. can be realized by the additions of a J K FFand some gates. Other types ofFF's may be used.

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AND8001/DTake the before mentioned Divide By 3 add a J K and a

divide by 6, 50% duty cycle, synchronous counter is realizedas shown in Figure 5.

~Jho 5Q D Q QA B C- - P K Of-Q - Q - CC C1 1 I

0% Out

Clk

Divide By 6 50% OutClkAQBQ

OUT, CQFigure 5.

Of course, there are better ways to realize a Divide By 6but it does demonstrate how the method works. Note thisconfiguration does not require a 50% input clock duty cycleand it is synchronous. This type of configuration could be

useful in a clock generating PLL chip where a Divide By 3and Divide By 6 are needed to synchronize two signals asshown in figure 6.



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AND8001/D

Clk

Divide By 3

~D 1 I r-- 50% OutQ D Q D Q D~A B C S = J- - Q r-Q f-- Q f-- QC C C1 I E Di50in

~K CQ

vide By 6% OutD-

ClK

AQ

BQ

DQ

OUT, EQFigure 6.

Notice FF "A" was chosen as the FF to drive FF "E" inorder to align the positive edges of the clock, Divide By 3,and divide by 6. The overall skew of the output could bebetter matched if all the same type of FF and gates are used.

We already know the Divide By 3 is lockup immune,following flow chart Figure 7 shows that the addition of theJ K does not change that situation for the Divide By 6.

Figure 7. Divide by 6 Flow ChartThe flow shows no lockup, but if one observes that the J The J K may need bigger input AND gates to accomplish

K is a sort of toggle device it is obvious that it can't lock up larger divide numbers. As an example, pick a Divide By 12the counter. and use J K type FF's to do the function.



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~J ~J5

-J ~J Q QQA B C D

~KK- K - K - QQ Q ~ Q r- -C C C C1 I I I1 1 I

Maps show:Ia = 1 JB =AC*Ka = 1 Kb =A Jc=ABKc=A

H

ClkJa = 1Ka = 1 Jb =ACKb =A

AND8001/D

Figure 8 shows the implementation.

0% Out

Jc = ABKc =A

Jd =ACDKd = ACD

ClK

AQBQ

OUT, DQFigure 8. Synchronous Divide By 12

A B C0 0 0 01 1 0 02 0 1 03 1 1 04 0 0 15 1 0 16 0 0 07 1 0 08 0 1 09 1 1 010 0 0 111 1 0 1

Examination of the truth table shows that the FF "D" mustdecode a 5 and a 13in order tomake the desire 50% function.The inputs to the "D" FF are J =ACD* and K =ACD and

requires 3 input AND gates. For larger counters the inputson the AND gates will need to increase to reach the desiredconfiguration; However for the single digit integers such as3,5,7, & 9 to realize 6, 10, 14, & 18a fan in of three is max.

Dooooo0* The truth table shows that the FF "D"

must change state at 5 and 13

1*

The methods are expandable. A little observation,thinking, and logic typing will allow the designer tominimize the component count and skew on this type ofcounter.

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AND8001/D

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