Barry Gershenfeld wrote:
>> Hello,
>>     I am in need to divide a 12.5 Mhz clock by 163. How can this be
>> done with a counter? I know that if this division were a power of
>> two, then it would be very straight forward, but this is not my case
>> at all. Any pointers will be greatly appreciated. -Vic
>
> If only it were an even number this would be straightforward.  But
> you had to pick 163.    Otherwise you divide by 2 at the end and
> the result is square.  I don't suppose you could fudge it to
> 162 or 164...
>
> Barry


There are some ways, including microcontroller counting pulses, but 12.5MHz
is a little bit high for an AVR executing NOPS for example, with an
intermediate flipping output port pin for an almost square wave.

1) First Solution
-----------------

You need to build a double polarity pulse discriminator, integrate into a
exclusive or, and get two sharp pulses for each complete 12.5MHz
squarewave.  You will have exactly 25MHz in pulse shape at the output of
the EOR.

Then you use the 25MHz to step two 74HC4020 and use the Q7 output to drive
a counting into a 74HC4017, use ouput Carry-Out (pin 12) to drive another
74HC4017, output pin 5 (Q6) will be your division by 163.

The only way to have a square output is to divide a frequency by an even
number, this is why you need first to multiply the frequency by 2.


                  +VCC
                    |
                   R01
                    |
            .---||--o----.   ____
            |            |__||   \
12.5MHz-----o             __||EOR O----> X
            |            |  ||___/       25MHz
            '---||--o----'
                    |
                   R02
                    |
                   _|_

           ___     ___     ___     ___
12.5MHz___|   |___|   |___|   |___|   |___

       ___ ___ ___ ___ ___ ___ ___ ___ ___
25MHz     |   |   |   |   |   |   |   |


         __________        _______        _______
        |          |      |       |      |       |
25MHz---|  4020    |Q7--->| 4017  |12--->| 4017  |5---> 76687.11656 Hz
        |__________|      |_______|      |_______|        _   _   _   _
             Dividing by 326                            _| |_| |_| |_|


2) Second Solution
------------------

      _________
     |         |
     |  4017   |Q3--.
     |_________|    |
          |         |      4023
      ___12____     |     ____
     |         |    |____|    \
     |  4017   |Q5_______|     o----o-----> 76687.11656 Hz
     |_________|     ____|     |    |
          |         |    |____/     |
12.5MHz---o         |               |
          |         |       <-------'
      ____|____     |      - Reset to
     |         |    |       All chips
     |   4020  |Q6--'
     |_________|

             Dividing by 163

But this solution generates a non square wave at output, simply a
negative fast pulse that is also used to reset all chips




/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
Wagner Lipnharski - UST Research Inc
Orlando FLorida - USA - www.ustr.net
/_/_/_/ Atmel AVR Consultant /_/_/_/

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu