CM8870 output (Q1-Q4) is 2V only. (Atmega16)

Discussion in 'The Projects Forum' started by alkgautam, Oct 2, 2016.

  1. alkgautam

    Thread Starter New Member

    Oct 2, 2016
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    Hello
    I m working on a project related to DTMF. But the 4-bits outputs have maximum of 2V. But when connecting to atmega16. Its not detected as 1.
    Because for valid one the input must be > 2.5V.
    Please help guys I m facing this problem from quite a long time.
    This is my circuit. AnimaCM8870.gif
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,519
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    Hi, and welcome to AAC.

    I might be wrong, but...

    1) Use transistors on each output as a switch. Then you'll need some extra circuitry.

    2) Use the atmega16 ADC function, and read the voltage with analogRead. With this option I do not think you need any additional circuitry.
     
  3. alkgautam

    Thread Starter New Member

    Oct 2, 2016
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    Thanks for the reply.
    for ADC function in the atmega16, Vref is only the input. Also the voltage at the output pins (Q4-Q3) is 2v(approx) const. so if I connect it with ADC function it will give only one value (0-255) corresponding to that reference voltage.
     
  4. nerdegutta

    Moderator

    Dec 15, 2009
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    With those values, is it impossible to tell the atmega that is has a valid input? A detection?
     
  5. alkgautam

    Thread Starter New Member

    Oct 2, 2016
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    Actually I need different 4-bits values i.e (Q1, Q2, Q3 Q4) output of the cm8870 and perform specific task as per the output through the micro controller. But due to low voltage its not detecting 1. for example the output is Q4(0), Q3(1), Q2(0) Q1(0) 1 corresponds to 2v and 0 corresponds to 0V. But to perform a specific task the microcontroller is detection all those inputs as 0. Because it will not detect 1 unless it is close to 3v or higher.

    Using ADC I will get only one value. also I can connect only one of the Q1, Q2, Q3 Q4. as Vref.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,179
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    What you are observing does not match the datasheet. If the behavior of the chip does not match the datasheet there is literally nothing you can do about it, except find a chip that does. Since this is an obsolete chip, it is unlikely that you obtained it from a reputable source. You either need to find a working chip or come up with an alternate solution.
     
  7. alkgautam

    Thread Starter New Member

    Oct 2, 2016
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    Actually I ve changed the IC several times its not working though. same problem everytime. Also in my region only cm8870 is avaliable not MT8870 which I could try. In the datasheet they say that Voh is 4.97V (max). But I m sill getting cm8870.PNG 2V :(.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Did you measure the output without a load?
    The datasheet says that it should be 4 Volts with a load of higher than 100K Ohms.

    Bertus
     
  9. alkgautam

    Thread Starter New Member

    Oct 2, 2016
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    I ve tried the circuit on a breadboard. The first thing I did was adding LEDs to check the output. Then I checked the output with led on. It was less than 2V. But whenn I removed the LED and checked again it was still 2V.
     
  10. Ylli

    New Member

    Nov 13, 2015
    26
    9
    Looks like you lifted the circuit directly from the California Micro devices data sheet, so I won't ask you about the circuit. Only thought would be to confirm that Vcc is actually at 5.0 v during operation.
     
  11. alkgautam

    Thread Starter New Member

    Oct 2, 2016
    7
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    yes I vcc is 5v. output from the 7805 voltage regulator.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    If you connected a LED directly to the output it is likely you damaged the chip. You said you changed the chip and it behaves the same. Seems like what you have is a bunch of chips that do not work for one reason or another. What is the purpose of the signal labeled StD? Could it be a Data Strobe? Would you ever expect the data outputs to be High-Impedance or Tri-Stated? How about showing us the full circuit instead of some cartoon version.
     
  13. DickCappels

    Moderator

    Aug 21, 2008
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    If your circuit matches the schematic and your device does not do what the datasheet says then as Papabravo said, either your datasheet is broken or your device is broken.

    But if you still desperate to see it work, you can try a level shifting circuit, which should be pretty simple given that it is a low speed signal. For example, you could try using a pair of dual voltage comparators to detect the signal level and convert it to the full swing necessary for the controller to "see" it. Alternatively you can try using an ATMEGA16L at 2.7 volts or an ATMEGA168 at 2 volts.

    By the way, do any of the outputs go to 4.7 volts or higher?
     
  14. alkgautam

    Thread Starter New Member

    Oct 2, 2016
    7
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    the StD just goes high whenever a valid dtmf tone is detected and the output pins are latched to the 4-bits binary. Or say it keeps blinking till u send a dtmf tone.
     
  15. Ylli

    New Member

    Nov 13, 2015
    26
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    Can you look at the outputs on a scope? Are they a steady DC value or are they pulsing high and low?
     
  16. Papabravo

    Expert

    Feb 24, 2006
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    If as you say StD is an output, then please tell me what allows the data lines to go into or out of a high impedance state?
     
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