| Electronics > Projects, Designs, and Technical Stuff |
| Is it safe to connect inputs to VCC directly |
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| wraper:
--- Quote from: james_s on February 05, 2020, 02:25:25 am --- --- Quote from: wraper on February 05, 2020, 12:13:40 am ---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. --- End quote --- How does tying unused inputs to Vcc or ground increase manufacturing cost and increase component count? --- End quote --- I thought you meant adding resistors as good practice. |
| tkamiya:
--- Quote ---A lot of modern "TTL" stuff is actually CMOS. Haven't seen anything claiming to be TTL. There might be written something like TTL voltage level. --- End quote --- Now I am genuinely interested in finding more. Taken literally, you are saying 74LSxxxx are actually CMOS with TTL interface. I am not finding anything on this. Will you please give me an example device so I can further research this? I have not seen anything like this. |
| oPossum:
74HCT and 74AHCT are CMOS with TTL thresholds |
| tkamiya:
Oh, OK. That I knew. I thought you guys were talking about 74LSxxxx series being redesigned as quasi TTL. |
| Miti:
--- Quote from: ataradov on February 05, 2020, 02:37:45 am --- --- Quote from: Miti on February 05, 2020, 02:33:21 am ---To kill a clock so the MCU doesn't start running the code. To keep an MCU in reset or otherwise it would interfere with your ICT or boundary scan. To pull up a "test enable" pin to put a JTAG compatible device in... well... test mode. --- End quote --- Those are provisions for dedicated pins. I'm not arguing with that. I'm asking specifically about single 74-series gates. Let's say I use 4-NAND and I only need 3 inputs. All I need is to force one other input to be always high. Any testing like that should happen through other 3 pins. This extra pin carries no function in the design. Unless you assign it a function to be a test input. --- End quote --- Yes, our customers with good design experience do this all the time, pulling input pins directly to GND or Vcc, even dedicated pins. Many that didn't design-for-test before, have an eureka moment when we ask them to add pullup/dn resistors in our DFT reports. |
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