8870 DTMF decoder - output latch

Discussion in 'General Electronics Chat' started by heena7, Feb 22, 2014.

  1. heena7

    Thread Starter New Member

    Dec 27, 2013
    There are 4 outputs of 8870 DTMF decoded digits. 1,2,3 or 4 pins will be high as BCD representation of digit.

    When new tone is detected, another set of output will be high.

    If one output pin is same in both output combinations, will it turned off momentarily and then turn on again or will it remain latched until new combination is active ?

  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    I'm pretty sure the 4-bit output buffer in the IC just changes from one binary state to the next, so it does not go to "zero" between changes. :)
  3. rogs

    Active Member

    Aug 28, 2009
    I'm not sure you could rely on all 4 latched outputs not going momentarily 'Hi Z' for some period of the tpStD interval (something less than 10uS, I would guess.
    The latch 'changeover' period between st/GT going high for a new valid code, and the StD flag going high to confirm latch update, shows an indeterminate (Hi-Z?) condition during that time period on the data sheet timing diagrams....
    I can't see the latch update logic being complex enough deal with individual outputs as being same or different, for the period (tpStD) that the data sheet has down as an 'indeterminate' latch state.......but I suppose it might be??..