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# substitute a d type flip flop for a j-k flip flop with j and k inputs tied to gnd an

#### handsomejackuk

Nov 12, 2011
12
can i use a d type flip flop

basically, i have this circuit to build, i mistakingly bought a d type flip flop 74hc 74 instead of the j-k type flip flop 74 hc 76

http://www.filedropper.com/bdm-if

<a href=http://www.filedropper.com/bdm-if><img src=http://www.filedropper.com/download_button.png width=127 height=145 border=0/></a><br /><div style=font-size:9px;font-family:Arial, Helvetica, sans-serif;width:127px;font-color:#44a854;> <a href=http://www.filedropper.com >file backup online</a></div>

now bearing in mind, that the j and k input of the 74 hc 76 are tied to vcc and gnd, what does that do ?

could i use my d type flip flop instead...?

other wise i have to make a 50 miles round trip to pick up a 74 hc 76

Al..

#### handsomejackuk

Nov 12, 2011
12
substitute a d type flip flop for a j-k flip flop with j and k inputs tied to gnd and vcc.....

i have this circuit to build, and only have a d type flip flop... will this work instead of the j-k flip flop, as both j and k are tied to gnd and vcc..

heres the circuit..

http://www.filedropper.com/bdm-if

Al..

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,506
OK, check the datasheet for the 74hc76. there is no connection to the clock on this device (the pins used are clear and preset -- CL is not clock!). You can determine what happens from the table near the top of this datasheet. strike out all lines that have anything to do with clock transitions, or where J and K are specified as other than they are hard wired. This will give you the logic.

Now take a look at this datasheeet. for the device you bought by mistake. Note that the clock is positive edge triggered and the other one is negative edge triggered. This would be important if the clock is used, but it's not!

The logic is a little simpler, but do the same as above, removing the lines with clock transitions. You will have one more line than is left in the diagram above. You need to determine which logic state the data pin needs to be tied to so that the Q-bar outputs are the same.

it seems that you should be able to make it work, but I've only glanced at the logic and not done the comparison that I suggest you do.

#### handsomejackuk

Nov 12, 2011
12
OK, check the datasheet for the 74hc76. there is no connection to the clock on this device (the pins used are clear and preset -- CL is not clock!). You can determine what happens from the table near the top of this datasheet. strike out all lines that have anything to do with clock transitions, or where J and K are specified as other than they are hard wired. This will give you the logic.

Now take a look at this datasheeet. for the device you bought by mistake. Note that the clock is positive edge triggered and the other one is negative edge triggered. This would be important if the clock is used, but it's not!

The logic is a little simpler, but do the same as above, removing the lines with clock transitions. You will have one more line than is left in the diagram above. You need to determine which logic state the data pin needs to be tied to so that the Q-bar outputs are the same.

it seems that you should be able to make it work, but I've only glanced at the logic and not done the comparison that I suggest you do.

Steve, thanks for the very quick replay, i will have another look tomorrow.. its getting late here almost 2.00 and its been a long nite... in u.k...
just a quick test of the printer port with the circuit plugged in reveals that the clock signal is logic high, on my old trusty multimeter.,..!!! . so that would agree with what you are saying, that the clock has no influence on the j-k flip flop either, just the set and preset are the governing inputs...

b.t.w. the circuit is for reading and writing ecu data to a saab ecu... if that makes it any easier to understand.... the connection on the right is a bdm interface....

hoping to remap my car with this... as it has been done before...

all good stuff...

thanks for your super quick help...

Al..

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,506
Damn. How did I miss that. Yes, the clock connection is connected.

Since a major difference between the device you want and the one you purchased is that they are triggered on opposite edges, you'll need to use that spare gate as an inverter to invert the clock input to the flipflop.

You will have to compare the lines with clock transitions too. A D flipflop essentially has an inverter connecting the J and K inputs, so since the J and K inputs are held at opposite logic levels, the D flipflop should work (remember that it won't be pin-compatible)

edit: hopefully I've uploaded that pdf so others won't need to go to that download site.

#### Attachments

• bdm-if.pdf
7.7 KB · Views: 260

#### handsomejackuk

Nov 12, 2011
12
Steve, thanks for that, i still a bit confuzzled whether or not i can use it, i may as well get the right chip, at least i know it will work then...

thanks for your help...

B.T.W. i did try and test it on a pc logic simulator, but its just to confuzzling for my small brain..!!!

unless anyone else can give me a positive yes...

Al...

#### handsomejackuk

Nov 12, 2011
12
anyone else know if this will work...?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,506
For a J/K flip flop, Q follows J, in a D flip flop, Q follows D.

So to use a D flip flop (in your case where J and K are tied to different supply rails) you need to tie the D input to the same place as the original J input.

If this is a duplicate post of the one I think it is then you also need to invert the clock as the JK flip flop is positive edge triggered and the D is negative edge triggered.

Moderator
Jan 21, 2010
25,506

#### handsomejackuk

Nov 12, 2011
12
thanks Steve, will give it a try...

yes it is the same post..

Al...

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