# how can i make BCD flip-flop down counter *urgent*

#### mohsen

Joined Oct 24, 2008
37
how can i make BCD series flip-flop down counter
i have made the counter count 0-9, how can i make it count 9-0

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#### Georacer

Joined Nov 25, 2009
5,182
Just for the record, this isn't a BCD counter. It is a binary counter hooked up on a Bin-to-BCD converter / 7-seg driver.

In order to count down from 9 to 0 and use only FFs to generate the numbers, you need some way to load number 9 paralelly at the start of each count cycle. But in an asynchronous circuit that doesn't seem possible.
If anyone has a different opinion, I would gladly hear it.

If I where you, I would consider using a synchonous counter like the "down counter" in this page http://www.allaboutcircuits.com/vol_4/chpt_11/3.html and find a way to implement a parallel load. This can be done with a few logic gates and four 2-to-1 MUXs. It can be a bit tricky, though. Ask further if you have more questions.

If this isn't a theoretical exercise and you just want to build a 9-0 counter, you can simply use the 74193 IC: http://www.datasheetcatalog.org/datasheets/90/248169_DS.pdf
It also contains a schematic for the whole system.

P.S. Does anyone know if the T-FFs in the datasheet are set with input T=1, and thus change state in every clock cycle? No direct reference on the FF input is made.

#### mohsen

Joined Oct 24, 2008
37
well i made this counter in the picture it count from 0-9 ^_^ so it is BCD lol

anyway i still don't get it how to make it count the otherway

#### shteii01

Joined Feb 19, 2010
4,644
Ok, since you can do the count up, 0 to 9. Can you put NOT gates at the outputs to invert the output? 0 becomes 9, and 9 becomes 0. The overall effect is count down.

#### Georacer

Joined Nov 25, 2009
5,182
If NOT gates are used to invert the 0-9 count, the result will be 1111,1110,1101,...,0101
Or in decimal 15-5. Clearly this is a no-go.

Are you not satisfied with the option of using the 74193 IC?