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Special king of one-shot circuit

Discussion in 'Electronic Design' started by [email protected], Apr 16, 2007.

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  1. Guest

    Hi,

    I'm working on a circuit that drives a termal printer.
    The printer has 8 output signals which activate the heaters.
    This signals connected to the CPU through a latch (74HC573).
    The sequence of the process is to enable the OE signal of the latch
    for about 2ms (at which the heaters are activated), then disable the
    OE and write new information to the printer via serial interface
    (takes about 100uS) and activate the OE again...

    I want to add a protection circuit that will recognize a situation
    that the OE of the latch is enabled for too much time (more then 4mS -
    that might burn the heaters) and will disable it.

    Of course, I'm looking for the simplest circuit possible.

    Any ideas?

    Thanks!
    Eyal.
     
  2. Robert Baer

    Robert Baer Guest

    Latch??? Why not a one-shot for the driver?
     
  3. Guest

    Robert Baer כתב:
    On-shot is not a good solution for a driver because the 2mS is
    software-controlled. it depends on the head temprature, blackness of
    the printing, paper type and more.
     
  4. Joerg

    Joerg Guest

    You could design the one-shot so it is "transparent". In other words so
    that the output disasserts when the software commands it to but also
    disasserts when 2msec have elapsed and the software failed to do so. For
    example by using a regular one-shot plus AND logic at the output.
     
  5. Guest

    Joerg כתב:
    I think that I have found the simplest solution.

    http://www.fairchildsemi.com/ds/CD/CD4538BC.pdf

    Thanks for your help.
    Eyal.
     
  6. budgie

    budgie Guest

  7. Guest

    budgie כתב:
    What is the "ST" version?
     
  8. Joerg

    Joerg Guest

    I guess he meant SGS Thomson.
     
  9. Or "ST Microelectronics", as they now call themselves. Why should their
    version be avoided? Just this chip, or the whole 4xxx series (or ST
    altogether)?

    robert
     
  10. Joerg

    Joerg Guest

    No idea what he meant with that. So far I've not had any bad experience
    with ST. <knocking on wood>
     
  11. budgie

    budgie Guest

    yep, ST Microelectronics aka SGS-Thompson.

    <background> Had a batch of ten products going through final test containing
    4017's from one of two manufacturers - ST Micro and On-Semi. Five out of the
    batch failed a particular test, which all previous batches had passed. These
    chips were driving devices with Moto MC14174B or Toshiba HD4174BP in a test jig.

    After some head-scratching, it transpired that all five failing units had
    HCF4017BE by STM while all the passing units were fitted with On-Semi. The STM
    chips were pulled and replaced with On-Semi parts, problem disappeared.

    I asked my supplier which brands comprised his current 4017 stock. His reply
    was: "4017 all stock is On Semi sorry about the problems. I generally don't get
    ST parts because of strange behaviours, the worst example is the 4538
    Monostable". </background>

    On seeing the reference to the 4538 in this thread, it flagged red to me.

    While one would be entitled to assume that an "industry standard" 4000-series
    chip can drive the inputs to another "industry standard" 4000-series chip, that
    appears to not always be the case.

    As always, YMMV, but I intend to steer clear of the ST branded stuff wherever
    possible.
     
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