L297 External Frequency to SYNC pin

Discussion in 'General Electronics Chat' started by edmundopt, Jun 28, 2011.

  1. edmundopt

    Thread Starter Member

    May 4, 2011
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    Hello

    I'm trying to input a frequency into the SYNC(1) pin of the L297 ic, the datasheet only mentions sync from other L297 but the idea is to use a 555timer from a complete isolated source!

    the schematic attached is only for explaining better, do not comment the optocoupler please, it's tested and it deals perfectly with high frequency!

    Has anyone done this before , use a 555 timer connected to SYNC pin ?

    thks
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Did you ground the OSC pin #16 on the L297 when doing this? This is from (http://www.st.com/stonline/books/pdf/docs/1334.pdf) page 4/11;
    "Ground noise problems in multiple configurations
    can be avoided by synchronizing the chopper oscillators.
    This is done by connecting all the SYNC
    pins together, mounting the oscillator RC network
    on one device only and grounding the OSC pin on​
    all other devices"

    The way your using the L297 is not the usual way. Is there a reason for doing it the way your trying? The Chip still needs the a frequency signal on pin #18 to make the motor turn.

    The SYNC pin and OSC pin only control the 'chopper' circuit that sets the amperage out-put.
     
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  3. edmundopt

    Thread Starter Member

    May 4, 2011
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    I think that's worth mentioning that I have working boards with the L297, with one providing the sync for the other 4, but I'm trying to improve them!

    Yes the OSC pin(16) is grounded, as specified in the data sheet!

    As I mentioned the schematic is only for explaining better.

    The CLOCK pin(18) is working very well, connected to an hex inverter(74hct04), connected to opto(6n137.

    The objective is to isolate the frequency source, and also control it beter with the 555, also avoiding ground loop
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    The frequency from the SYNC pin is generated internally in the L297, you would have to figure a way to disable the whole chopper circuit to add an external one. Can't really see an advantage to doing that. Plus it would mean adding more components to the circuit.
     
  5. edmundopt

    Thread Starter Member

    May 4, 2011
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    maybe my english is not so good, let me try to make the question in another way:

    is it possible to inject a frequency from 20khz to 40khz using a 555 into the SYNC pin of the L297?
    would it work the same way that the R/C that connects the OSC pin?
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    It would work, but there is NO advantage. You would need a 555 circuit to feed EACH of the L297's.

    They(manufactures of the L297) went to a lot of trouble to include the chopper circuit in the chip. The SYNC pin(stands for synchronize) is there to allow ALL of the linked chips to work under the same conditions. It also allows only ONE set of components to be needed on the master chip to control all of the slaved chips connected to the master.

    The OSC(oscillator)pin is basically a internal 555 chip. By setting the frequency on that pin you are controlling the chopper circuit(Amperage control to the motor windings). If you want to operate your chopper circuit at 20 - 40 kHz just use component values to generate that frequency there.

    Have you read the data sheet for the L297? http://www.st.com/stonline/books/pdf/docs/1334.pdf The workings of the chip are on page 4/11. The pin functions are on pages 3/11 and 4/11. A drawing of how the chips are synchronized is on page 8/11, figure 3.

    If this doesn't help I still don't understand why you want to operate the chips the way you want.
     
  7. edmundopt

    Thread Starter Member

    May 4, 2011
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    Thank you for your reply!

    I was thinking of one 555 for all the boards, since the opto chip is the one providing the frequency.

    I think I have a ground loop effect on the boards making some noise in the steppers(change the RC values on OSC pin from 20khz..40khz did not work), since each board has it's own 5v and it's own 44v, the only wire that was common was the sync, each board works perfectly, but all boards working, with sync( or not) make the noise, the idea was to eliminate the common ground!

    Anyway, thank you for your reply!
     
  8. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    This is from the last paragraph on page 4/11 of the link I posted-
    "Ground noise problems in multiple configurations
    can be avoided by synchronizing the chopper oscillators.
    This is done by connecting all the SYNC
    pins together, mounting the oscillator RC network
    on one device only and grounding the OSC pin on​
    all other devices."

    The only other frequency you need is the one into pin #18 - this HAS to be a square wave. Each time the square wave rises the motor move one step.

    What is causing you to think that you have a "ground Loop" ? This is not a normal thing for some thing like this.

    I know you didn't want comments on your opto schematic, but, if thats how you actually wired it ,not to good. Maybe post your whole schematic?
     
  9. edmundopt

    Thread Starter Member

    May 4, 2011
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    Hello shortbus

    I found that changing the R and C value of the OSC pin, using different values for each board works better than using the sync pin, the noise disappears..

    For the complete schematic,including an explanation about the optocoupler, and I'm actually looking for suggestions, please visit :

    http://forum.allaboutcircuits.com/showthread.php?p=377616#post377616
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    You never stated in any of your posts that you were trying to synchronize circuits on separate boards.
     
  11. edmundopt

    Thread Starter Member

    May 4, 2011
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    I believe that's because of my bad English.
    I've 5 boards, I was trying to use the SYNC(1) output pin from one, and connect it to the other 4 boards SYNC(1) pin with the OSC pin(16) grounded.

    When connected this way, there was noise that I want to eliminate.

    Right now, they work better with each board using different R and C values, making a different frequency for each board and not using the sync pin, sync should be better, in my case it just isn't, please take a look at the board in the other post :) and sorry for my English
     
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