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Why do 7490 and 7493 have gated reset inputs? For example: 7493 has two inputs MR1 and MR2. Internally, these two inputs are fed into a nand gate, the output of which is use as the reset ( active low ) input for all the four flip flops.

Wouldn't it have been better if there is just one input (say, MR ) which will reset all the flip flops?

Internal Circuit Diagram of
7490 : link
7493 : link

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No, it wouldn't have been "better". You can always tie off any unused gate inputs so that you have a single control input, if that's all you need.

The gates are there so that the counters can easily be wired to divide by factors other than their "native" values (modulus of 10 for 7490, 16 for 7493). With a two-input gate provided on the same chip, you can divide by any modulus other than 8, 12 or 15, each of which would require a 3-input gate to detect the final values of 7, 11 or 14, respectively.

The choice of using two-input gates for this was driven by a number of issues, including the ease of adding the feature to the die, the number of pins available on the package, and the level of convenience the feature added to general usage of the chips.

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