Up counter?

Question

What is 4-bit asynchronous UP counter means?(digital logic)

Answer 1

It is a counter, that has 4 binary bits, that counts from 0000 to 1111 (decimal 0 to 15), in the up direction (1, 2, 3, 4... )
It does not in the down direction (9,8,7,6...). The count changes simultaneously for each of the 4 bits of the binary count on the edge of the clock input -- usually the rising edge of the clock.

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Answer 2

asynchronous means that the bits do not all change at the same time, which can cause problems in some circuits - normally this means that the lowest bit changes and the changes cascade up through the bits (like carrying in ordinary addition)
when a chip is synchronous various methods are used to present all the bits, already changed, together.

Answer 3

these guys got the answers right already.

in the following positions:

8421
-------
0000 = 0
0001 = 1
0010 = 2
0101 = 5
1111 = 15