Clock Dividers

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. In this post I am going to talk about how to divide a clock by an odd number.

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. Because of the constant 90° phase offset, only one transition occurs at a time on the input of the exclusive-OR gate, effectively eliminating any glitches on the output waveform.

Let’s see how it works by taking an example where the REF_CLK is divided by 3.

  • Create a counter which is incremented on every rising edge of the input clock (REF_CLK) and the counter is reset to ZERO when the terminal count of counter reaches to (N-1). where N is odd number (3, 5, 7 and so on)
  • Take two toggle flip-flops and generate their enables as follows; T-FF1 is enabled when the counter reaches '0' and T-FF2 is enabled when the counter reaches (N/2)+1.
  • Output of T-FF1 is triggered on rising edge of REF_CLK and output of T-FF2 is triggered on the falling edge of REF_CLK.
  • The divide by N clock is derived by simply Ex-ORing both the output of T-FFs.

The above Figure shows the timing diagram for the above steps.

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