I am studying for my digital electronics final and one of the major topics is flip flop counters. I need to know how to get the asynchronous counter to preset to 2 before counting to 10. I know how to make it reset (With some well placed NAND gates), but I cant figure out how to get the counter to preset to 2 after reseting. Any help would be much appreciated. Thanks!
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\$\begingroup\$ are you building your FF's out of discrete gates? \$\endgroup\$– JustJeffCommented Jun 9, 2010 at 21:19
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\$\begingroup\$ No I am using just JK ICs. \$\endgroup\$– HristosCommented Jun 9, 2010 at 21:37
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\$\begingroup\$ are you saying you want it to count from 2 to 10, and then roll over back to 2? \$\endgroup\$– vicatcuCommented Jun 10, 2010 at 2:43
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\$\begingroup\$ Exactly Vicatcu \$\endgroup\$– HristosCommented Jun 10, 2010 at 10:39
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1 Answer
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Common JK flip flops, such as the 7476 have both preset and clear inputs. These force the state of the FF asynchronously, i.e., without waiting for a clock edge. You can use a simple R-C circuit and a schmitt trigger buffer (or inverter) to give you a short (e.g., 50usec) active-low pulse at circuit power on, and feed this signal to the 'preset' inputs of the FF's that should start at '1', and to the 'clear' inputs of the FF's that need to start at 0. (The unused preset or clear of each FF should be pulled up to prevent erratic operation.) The spec sheet for the particular part will tell you how short this pulse can be made.
