• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 18 subscribers
  • Views 594 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

74AHC08 outputs wired to combined inputs, error in logic

Robert Lewis
Prodigy 100 points

I thought this was a simple circuit but I am not getting correct logic. We have four gates connected to produce a single output if any one of 5 inputs goes low. This is powered at 3.3V.

1,2 are inputs from switch     ( 1 & 2) --> 3

4,5 are inputs from switch     ( 4 & 5) --> 6

                                                    (6 connects to 9) & (3 connects to 8) --> 10

12 is an input from switch      (10 connects to 11) & ( 12) --> 13, 13 is always low

 

where the inputs on pins 1,2,4,5,12 are from a momentary contact to ground, 3,6, 10 are logic outputs of the AND gates. Therefore the idea was if any one of the inputs goes low we will get a low on 13. But we don't, 13 is always low and some of the other outputs miss an input transition. We have tried multiple boards so I don't think we have a bad part, and we have looked through the board build and there are no shorts. I think there is something I am overlooking wrt the load or something else and would appreciate any help. Thank you.

 

 

 

  • 0
    Prodigy 100 points

    Update to the above. I think I have a fundamental error; the inputs of the ANDs are also connected to the ports on a micro, these inputs are set to be inputs with a pull up resistor. But I just realized that the pull up is an internal circuit and as such isn't really the same as a pull up as far as the AND gate is concerned, so the inputs are effectively floating. I added a set of external resistors to the inputs of 4.7k to Vdd but the gate still does not function correctly. I think the gates are damaged.

    I haven't worked with CMOS that much so here is the question, can the gates be damaged because I didn't pull them up with external resistors. That is the switch was pulling the inputs low but there would not have been a current source from the high impedance input of the micro port? Any advice would be appreciated. Thanks

     

    Robert

  • 0
    TI__Mastermind 46480 points

    Can you Email me a schematic?

    c-cockrill@ti.com

    Thanks

  • 0 in reply to Robert Lewis
    TI__Mastermind 46480 points

    The gates will need a good High and a good low. they should not float or they could be damaged. Also the momentary switches should have some type of debouncing circuit.