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I need to make mod-12 asynchronous down counter using T flip flop. It counts from 15 to 4. I don't know really how to do it and it's really important, so if you could help me i would be very grateful!
You are trying to build a counter that has exactly 12 states. Those 12 unique states can be named anything, including 15,14,12,...,5,4. When you build a counter out of "state elements" (flip-flops), you can arrange them in many ways. You should strive to arrange them so that the numerical value of the states as defined by the binary pattern of the individual flip-flops matches the numerical value such as 15=1111, 14=1110, ..., 0101=5, 0100=4.
With this in mind, you should be able to create a "present state - next state" table.
Question for you: How many flip-flops does it take to represent 12 unique states?
i agree . . . but is it a solution ? . . . i barely got it work coz the maladies of a simplistic simulator
+ in case it verifies to a solution - ? is it optimum or what there should be (as expected by course master ?)
it initalizes "right" only by pure chance (by the specifics of the simulator)
. . . it is not a solution but rather a warning what everything might go wrong while attempting to find one
when it comes to a dynamic circuit elements such as triggers the output of such is not a fixed 1 to 1 accordance but also it's internal state
this may complicate keeping track on events and to design your sys - so far you've done all actually correct (not getting lost in the time sequence)
also right is that you have defined the output as the function of input
you haven't told us not shown the particular T-flops you have to implement this function on
? synchronous (with the C (clock) input) async. (T input only) -- the function you try to achieve depends on that
There are many different ways to treat states 3 to 0. If you use them as don't cares "X" in the state table, it might simplify the gating required to implement the counter, but then you really need to test for "hazards" after you are done.
I once designed a counter that went into a commercial product where I created a hazard short loop. Occasionally, on power-up, the flip-flops would come up in one of the "dont-care" states, but the hazard was that the counter had two stable counting patterns; the desired one and the undesired shorter one. You need to verify that the next state for states 3 to 0 lead to one of the "legal" states. My boss was not happy...
as the input T-high allows "count" (output swap) and the T-low disables it
you set your T by previous output of all Qx -s using boolean arithmetic operations (implemented on digital circuitry)
the breakout from undesired loops is achieved by setting up an additional control to some of the Tx -s
(which is activated when the triggers output is at undesired range) - versus - you initialize your triggers to a
"legal" output (again by setting up initialization cycle or synchronously driving the Reset Set inputs if they are implemented)
the synchronous by additional memory-/RAM-/EEPROM lookup-table
potentially the fastest (depending on memory type)
the same as above but your digital T-driver functions is stored statically in RAM/ROM adressed by "counter output"
the async. by logic circuitry - where each prev (trigger/couter stage →) Qx defines* the next Qx+1
(there will be time delay before MSB-s set by/after LSB-s do)
has a perhaps a more complex implementation than the ones above <not described here - if you can do above then you can figure out and this>
the async./to sync. counter -- by setting T→ input aside as an async. clock →C
and implementing syncrounous inputs for that new "clock" as in "1."
(sh¡t - i don't remember already) . . .
if avail than using RS inputs to handle "**non-trivial" state transition
by disabling/+redirecting the T-/clock- (see. "4.") input for such** transition
and precisely driving RS inputs to achieve desired output state ...
All this requirement has died 10 years ago.
Counters use a microcontroller and this allows up and down counting, false triggering, setting a count, creating an alarm, and lots of other features with a single chip costing 50 cents! Look at what the Chinese have !!!!! STC15W201S