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I want to make a digital timer that counts to 99 minutes 59 seconds the problem is that i want to use 4 digit 7 segment display so i have to oscillate the input of the bcd to 7 segment display between the four digits so i have to control the decade counter 74160 so i use mux to choose which counter to work and counter 5 oscillate switches the mux quickly . How to control f/f wheather to output or to keep the output for itself for now till it`s turn? This is a quick schematic for the criteria

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  • \$\begingroup\$ Why don't you use a sim tool like micro-cap (free) and work this out yourself? \$\endgroup\$ – Andy aka Dec 18 '19 at 9:18
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To do this with minimal changes to your design, consider using a tri-state buffers at the output of each counter.

This is a buffer with an ENable pin. When the enable pin is asserted, the buffer produces an output like normal. When the enable pin is de-asserted, the buffer puts its outputs in high-impedance (or high-Z) state. This not only means there's no output, it also means the output won't interfere with other gates trying to produce outputs onto the same wires.

For example, consider 74LS244. This has two sets of 4 tri-state buffers, with each set of 4 controlled by a common enable pin. Note the enable pin is active-low, so "asserted" means a low voltage and "de-asserted" means a high voltage. You'd need two of these to complete your design.

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