74HC595 connect to Vcc

[shobo]

Why is the SCL line on a 74HC595 connected to Vcc (i guess the more correct terms would be pulled high)?

i took inspiration from this instructable: http://www.instructables.com/id/Arduino-powered-7-seg-LED-display-using-Shift-Regi/ to design a display board for a soldering station i intend to build somewhere in the future.

[rstofer]

Because a) signals can't be allowed to float and b) the signal is active low and it shouldn't be active.  I would never tie a pin directly to Vcc instead I would use a 1k resistor.  Opinions vary on this but I have never liked tying signals to a solid Vcc.

[David Hess]

Because a) signals can't be allowed to float and b) the signal is active low and it shouldn't be active.  I would never tie a pin directly to Vcc instead I would use a 1k resistor.  Opinions vary on this but I have never liked tying signal to a solid Vcc.

Tying a logic input to Vcc matters where TTL (transistor transistor logic) that uses emitter inputs is used.  The input transistor will conduct current in reverse mode limited only by reverse beta and input resistance.  Later TTL families use diode inputs which do not suffer from this problem.

This is not a problem for HC or any CMOS logic.

[2N3055]

That pin is Shift Register Clear. It resets contents of shift register. It is active low. If you don't want to use reset, tie it to logic high and ignore it.
Regards,
Sinisa

[rstofer]

Because a) signals can't be allowed to float and b) the signal is active low and it shouldn't be active.  I would never tie a pin directly to Vcc instead I would use a 1k resistor.  Opinions vary on this but I have never liked tying signal to a solid Vcc.

Tying a logic input to Vcc matters where TTL (transistor transistor logic) that uses emitter inputs is used.  The input transistor will conduct current in reverse mode limited only by reverse beta and input resistance.  Later TTL families use diode inputs which do not suffer from this problem.

This is not a problem for HC or any CMOS logic.

I'm just old school, I guess.  However, if I build a PCB and connect the pin to the Vcc plane (inside a 4 layer board), I can never change my mind (short of cutting off the pin).  OTOH, if I connect the pin to Vcc through an SMD resistor, I can add a bodge wire and regain control of the signal.  I can choose to keep or remove the resistor.


You're right, this admonition goes back to the early days of TTL.

[2N3055]

I'm just old school, I guess.  ....

Nah, that's just smart... I do that or at least run a short trace before diving into layers.. So I have something to cut in case I need it....

Regards,

Sinisa