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Subject: Synchronous Counters
Date: Sat, 1 Jan 2011 15:23:44 +0200
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD><TITLE>Synchronous Counters</TITLE>
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<H1 align=3Dcenter><FONT color=3Dpurple><I>Synchronous =
Counters</I></FONT> </H1>
<BLOCKQUOTE><B>In <FONT color=3D#ff0080><BIG><I>synchronous=20
  counters</I></BIG></FONT>, the clock inputs of all the flip-flops are=20
  connected together and are triggered by the input pulses. &nbsp;Thus, =
all the=20
  flip-flops change state simultaneously (in parallel). &nbsp;The =
circuit below=20
  is a <FONT color=3D#0080ff>3-bit synchronous counter</FONT>. &nbsp;The =
J and K=20
  inputs of FF0 are connected to HIGH. &nbsp;FF1 has its J and K inputs=20
  connected to the output of FF0, and the J and K inputs of FF2 are =
connected to=20
  the output of an AND gate that is fed by the outputs of FF0 and=20
FF1.<BR></B></BLOCKQUOTE>
<P align=3Dcenter><IMG border=3D0 alt=3D"3-bit Synchronous Counter"=20
src=3D"http://www.eelab.usyd.edu.au/digital_tutorial/part2/pics/count05.j=
pg"=20
width=3D422 height=3D135>=20
<BLOCKQUOTE>
  <P align=3Dleft><B>Pay attention to what happens after the 3rd clock =
pulse.=20
  &nbsp;Both outputs of FF0 and FF1 are HIGH. &nbsp;The positive edge of =
the 4th=20
  clock pulse will cause FF2 to change its state due to the AND=20
gate.<BR></B></P></BLOCKQUOTE>
<P>
<CENTER><IMG border=3D0 alt=3D"Timing Diagram"=20
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<P>
<TABLE cellPadding=3D2>
  <TBODY>
  <TR>
    <TD><IMG border=3D0 alt=3D"Truth Table"=20
      =
src=3D"http://www.eelab.usyd.edu.au/digital_tutorial/part2/pics/count05t.=
jpg"=20
      width=3D149 height=3D160></TD>
    <TD><B>The count sequence for the 3-bit counter is shown on the =
right.</B>=20

      <P><B>The <U>most important advantage</U> of synchronous counters =
is that=20
      there is no cumulative time delay because all flip-flops are =
triggered in=20
      parallel. &nbsp;Thus, the maximum operating frequency for this =
counter=20
      will be significantly higher than for the corresponding ripple=20
      counter.</B></P></TD></TR></TBODY></TABLE></CENTER>
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