Pull-up resistors on 74HC series

[ABCD]

I have a 74HC112N, dual negative edge triggered JK flip flop with preset and clear.  I want to set it to toggle mode, which I know how to do in terms of what the inputs need to be, but when it comes to wiring it up, I'm not sure how to configure the inputs.  They can't be left floating, I know, and the datasheet says to tie unused inputs to either Vcc or GND.  But it makes no mention of pullups.  So, can I just wire J and K to Vcc, PRE and CLR to Vcc (they are active LOW) and be OK?  (I'd tie all inputs for the second FF to Vcc I suppose.)  My impression is that if I attached switches to J and K, I'd need to use pullups to keep the inputs at logical 1 (or pulldowns for logical 0) so that I don't short out the pins when I push the button.

[tecman]

You can tie any input hi or lo, as required, without any problem.  Pull ups not required.

paul

[ABCD]

Thanks, that's what I thought from reading the datasheet.  If attached switches to inputs J and K, would a pullup then be needed?  I am still a little unclear as to when they need to be used.

[Zero999]

I think you nedd to lead a bit more about what a pull-up/dwon resistor actually does so you can answer your own question.

A CMOS gate has a high impedance (>10M) which can float at any voltage if not connected to either power supply rail.

Now look at the internal structure of a typical CMOS gate.


If V_IN is half way between V_DD and the negative rail, both Qn and Qp will turn on causing a current to flow which would cause the output voltage to also flow between V_DD and the negative rail. The result is your CMOS IC will use much more power than it's supposed to.

When the gate is working properly and is static either Qn or Qp will be on, not both which should only happen when the input voltage is changing from high to low or vice versa.

A pull-up/down resistor is only required when there's a chance a CMOS input will not be connected to either V_DD or the negative rail.

[ABCD]

Hero999-

Thanks, I believe I have the concept down now.