Clock Dividers Made Clock Dividers Made EasyEasy

WHAT IS A CLOCK DIVIDER ?WHAT IS A CLOCK DIVIDER ?

A CLOCK DIVIDER or FREQUENCY DIVIDER   is an  electronic circuit  that takes an input signal with a frequency ,  fin, and generates an output signal with a frequency:

where n is an integer.

WHAT IS 50 % DUTY CYCLE ?WHAT IS 50 % DUTY CYCLE ?

the duty cycle is the fraction of time that a system is in an "active" state.

In a periodic phenomenon, the ratio of the duration of the

phenomenon in a given period to the period.

duty cycle 

where

is the duration that the function is  active high (normally when the signal is greater than zero);

Τ is the period of the function.

τ

WHY 50 % DUTY CYCLEWHY 50 % DUTY CYCLE

Duty cycle tells the time intervals devoted to starting, running, stopping, and idling when a device is used for intermittent duty.

 It tells a machine's rated capacity to continuously perform work under normal conditions. It generally applies to mechanical devices such as printers, in which case it would indicate the number of pages that can be printed per month without a problem.

In engineering :

 The ratio of a device operating versus cooling down. An 80% duty cycle means that it is capable of running 80% of a specified time period and turned off for the remainder.

In radar engineering :

Duty cycle (or duty factor) is a measure of the fraction of the time a radar is transmitting.

In metallurgy :

Duty cycle gives the percentage of time that current flows in equipment over a specific period during electric resistance welding.

Dividing a clock by an even number always

generates 50% duty cycle output.

Sometimes it is necessary to generate a 50% duty cycle frequency even when the input clock is divided by an odd or non-integer number. This paper talks about implementation of unusual clock dividers.

Odd integer division with Odd integer division with 50% duty cycle50% duty cycle

The easiest way to create an odd divider with a 50% duty cycle is to generate two

clocks at half the desired output frequency with a quadrature-phase relationship (constant 90° phase difference between the two clocks).

You can then generate the output frequency by exclusive-ORing the two waveforms together.

STEP I: Create a counter that counts from 0 to (N –1) and always clocks on the rising edge of the input clock.

STEP II: Take two toggle flip-flops and generate their enables as follows:

tff1_en : T FF1 enabled when the counter value = 0

tff2_en : T FF2 enabled when the counter value = (2 for Divide by 3 counter , 3 for Divide by 5 counter, 4 for Divide by 7 counter and likewise).

STEP III : div1 : output of T FF1 triggered on

rising edge of input clock (ref_clk). div2 : output of T FF2 triggered on

falling edge of input clock (ref_clk).STEP IV : Final output clock: clkout (Divide by

N) is generated by XORing the div1 and div2 waveforms.

Non-integer division (duty Non-integer division (duty cycle not 50%)cycle not 50%)

Alternative approach for Alternative approach for divide by-N divide by-N

All the circuits use combinatorial feedback around a LUT (look up table)

that works perfectly

Divide by 1.5 Divide by 1.5 ( LUT implementation) ( LUT implementation)

CONCLUSIONCONCLUSION

The circuits are simple , efficient and are cheaper and faster.

In case of non-integer clock dividers (divide by 1.5,divide by 2.5 etc) one cannot get anything better than 40 % - 60 % duty cycle with a digital circuits.

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