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JK flip-flop as toggle, default power on state???

james lawrence
Prodigy 10 points


Hi there,

I am trying to use a JK flip-flop neg edge trigger (CD74AC112M), as a toggle o/p from a momentary switch. As I read the datasheet if you tie J, K,  !PRE & !CLR high and put a low edge on the clk pin the o/p Q toggles. This works as intended, however it is being used to turn on a PSU from a small LDO and the problem is it can power up in high or low state.

Can anyone help please? I understand you can use a Microcontroller monitor IC to initialise the output state, but have had no luck to date.

Thanks

James

  • 0
    Guru 22270 points

    Hello James,

    One way to get the CD74AC112M to be in the desired state at start up is to use weak pull up/down resistors on the inputs. 

    You could go with the configuration of !PRE tied high, !CLR + J + K tied to CLK.  A high on the CLK + J + K will set the output to High and a low CLK will toggle it.

    Thanks