Electronics > Projects, Designs, and Technical Stuff

Is it safe to connect inputs to VCC directly

<< < (5/14) > >>

wraper:

--- Quote from: james_s on February 05, 2020, 12:05:04 am ---It's just good practice to do so, it can prevent strange problems.

--- End quote ---
More like stupid practice how to increase manufacturing costs and problems due to higher component count. Also you reduce noise immunity or robustness against board contamination or tin whiskers. Say CMOS output (connected to input) has much lower impedance than a few k resistor you would use as pull-up.
EDIT: It was about adding resistors. Apparently james_s meant connecting to power rail.

--- Quote ---A lot of modern "TTL" stuff is actually CMOS.
--- End quote ---
Haven't seen anything claiming to be TTL. There might be written something like TTL voltage level.

MasterT:

--- Quote from: ataradov on February 04, 2020, 11:29:24 pm ---But what does SCR latch-up has to do with connecting inputs to the VCC directly?

--- End quote ---
In normal condition doesn't matter if there is a resistor installed or not. But a good designer have to predict all abnormal situation as well, like:
1. over-voltage to the IC
2. ESD discharge
3. current injection to/from neighboring inputs/ outputs that may get signal from other modules/ boards etc.
4. power pin connection loss
5. strong RF pulse - EMP warfare.

Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .

iMo:
It is a good practice, originated in times where a 2 layer pcb board was 40x80cm large with 500 DIL14/16 chips on it. On such a board with a dozen of XXcm long parallel straight Vcc and Gnd rails, following the never ending rows of DIL sockets, the term "connected directly to Vcc or Gnd" may not always mean "connected directly to Vcc or Gnd".. :)

ataradov:
 As I said, I'm not interested in catastrophic failures. If there is over-voltage or power pin connection loss, then device is broken. It does not really matter what happens after that.

I don't buy this "good designer" stuff.

I see this matter for aerospace or military applications, but there you have to think about much more than just a simple pull-up.

wraper:

--- Quote from: MasterT on February 05, 2020, 12:30:02 am ---Inputs are not designed to pass much current, so there is "probability"  latching-up, and it's easier to install a resistor than estimate 12-20 inputs/outputs interference to each other .

--- End quote ---
Again, please explain how MOSFET gates tied to Vdd/Vss are supposed to pass any current. Latch-up does not result in current passing through the gates.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod