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Thread: 16x2 LCD showing two lines of Boxes

  1. #11
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    I found that some LCDs do not like having the unused data pins connected when they are used in 4 bit mode. Not sure why, but I have one project where the display didn't work on the finished PCB and I eventually traced it to the unused data pins being tied to ground. A craft knife slice though the tracks on the finished PCB fixed that! When testing it on the breadboard I'd not bothered to tie the unused data pins down, so the thing worked, but when making the PCB I thought it would be good practice if they were just tied low. IIRC, the data sheet for that LCD said that the state of the unused data pins in 4 bit mode was "don't care", which was misleading.

  2. #12
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    Just checking your circuit diagram. Possibly not the cause of your immediate problems but you need to put a 100nF (0.1uF) capacitor close to the PICAXE's power pins. Also, the DS18B20 needs a pull-up resistor on the data line.

  3. #13

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    On my circuit itself, the DS18B20 does have a 4k7 pull-up resistor on the data lines. I just remembered that when I breadboarded the design, the Potentiometer had fallen out and was not present when the circuit began to run. There was no fixed resistor in its place, just a variable power pack that I was using instead of batteries at the time.

  4. #14
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    Not near a computer at the moment, but one issue I see with the schematic is that the output from the 20M2 to the lcd firmware has a serial 1k resistor, with yet another 1k resistor from that connection location to ground. Fairly certain that will cause only around half rail signals to the firmware IC. Fairly certain that the firmware IC won't like that arrangement unless it can work at lower logic levels than the 5V supply. Given it's working off the 5V rail, I doubt that is going to work reliably.

    Try removing the 1k to ground. If one is specified for the LCD, one would expect at least a 10k resistor. To avoid a potentially (significant) voltage divider, reduce the series resistor from the 20M2 to the firmware IC to around 100 ohms (protects against a total dead short) and, if needed, make the pulldown resistor around 10k.

    Not sure how it ever worked reliably with a half rail/logic divider???

    Hope that helps.
    Darb

  5. #15
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    Quote Originally Posted by darb1972 View Post
    one issue I see with the schematic is that the output from the 20M2 to the lcd firmware has a serial 1k resistor, with yet another 1k resistor from that connection location to ground.
    Good catch. The FRM010 datasheet shows how the circuit should be -

    Code:
                       __
    PICAXE |------.---|__|--->| FRM010
                 .|.
             10K |_|   1K
                 _|_
    That's the same configuration as the PICAXE download interface. The 1K for current limiting, the 10K to keep the input signal low when nothing connected. It is not a voltage divider.

  6. #16
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    Quote Originally Posted by hippy View Post
    Good catch. The FRM010 datasheet shows how the circuit should be -

    Code:
                       __
    PICAXE |------.---|__|--->| FRM010
                 .|.
             10K |_|   1K
                 _|_
    That's the same configuration as the PICaxe download interface. The 1K for current limiting, the 10K to keep the input signal low when nothing connected. It is not a voltage divider.
    True Hippy. If the 10k pulldown is placed before the 1k then it won't act as a voltage divider as the PICaxe is the source. But, if the 10k was placed after the 1k (closest to the FRM10) then it would act as a (somewhat minor) voltage divider.

    Thanks for clarifying the correct position/configuration of the resistors.
    Last edited by darb1972; 07-01-2018 at 20:42. Reason: Typo
    Darb

  7. #17

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    Quote Originally Posted by darb1972 View Post
    True Hippy. If the 10k pulldown is placed before the 1k then it won't act as a voltage divider as the PICaxe is the source. But, if the 10k was placed after the 1k (closest to the FRM10) then it would act as a (somewhat minor) voltage divider.

    Thanks for clarifying the correct position/configuration of the resistors.
    I checked this with a 10K resistor but still no change. When you said it is the same as the download layout, I tried a 22k(that's what I am using from download socket to the 20m2) to see if that did anything, but still not working.

  8. #18
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    Did you move the 10K/22K resistor to before the 1K, to be on the PICAXE output rather than the FRM010 input ?

    The resistor may be the issue but it's not the only possibility. That everything worked on breadboard but not on the PCB indicates there's been some change in turning what worked into the circuit which is on the PCB.

    In your earlier post you said you weren't sure the circuit you posted matched with what you had on the breadboard.

    Could it be you never made the changes to the circuit which you made to the breadboard to get it to work, created the PCB from the originally drawn circuit, not the breadboard circuit ?

    You have two boards but only one circuit. It's not clear exactly what is on each board.

    I think it's just a matter of comparing board tracking to what it should be but that's not easy to do without clear photos showing the tracking and the boards you have seem to have had a number of mods applied which makes it harder to tell what you have.

    You may have to resort to checking every solder point on the board, checking they do go to every other solder point they should go to, do not go to any others.

  9. #19
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    odj233,

    I checked this with a 10K resistor but still no change
    I've spotted a second item that looks wrong in your schematic.

    As hippy has already pointed out, the section of circuit for the serial input to the FRM010 is given in the FRM010 datasheet like this:
    Code:
    PICAXE |------.---|__|--->| FRM010
                 .|.
             10K |_|   1K
                 _|_
    and this is the serial input section of the circuit taken from the schematic that you posted :
    Code:
    PICAXE |---|__|-----.---->| FRM010
                       .|.
             R8 1K     |_| 1K R9
                       _|_
    Your serial input circuit has two differences:
    - as hippy pointed out, you have the R9 resistor to ground as 1K, where the frm010 manual specifies this as a 10k resistor
    - I've just noticed that you also have put the R8 1K resistor on the "PICAXE" side of your circuit where the FRM010 manual has this 1K resistor on the "FRM010" side of the circuit.

    I tried a 22k
    This is the circuit for the serial input to pin 3 of the FRM010. It is not a serial download circuit so the resistor values from the FRM010 manual are the ones that will apply.

  10. #20
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    Quote Originally Posted by Flenser View Post
    odj233,



    I've spotted a second item that looks wrong in your schematic.

    As hippy has already pointed out, the section of circuit for the serial input to the FRM010 is given in the FRM010 datasheet like this:
    Code:
    PICAXE |------.---|__|--->| FRM010
                 .|.
             10K |_|   1K
                 _|_
    and this is the serial input section of the circuit taken from the schematic that you posted :
    Code:
    PICAXE |---|__|-----.---->| FRM010
                       .|.
             R8 1K     |_| 1K R9
                       _|_
    Your serial input circuit has two differences:
    - as hippy pointed out, you have the R9 resistor to ground as 1K, where the frm010 manual specifies this as a 10k resistor
    - I've just noticed that you also have put the R8 1K resistor on the "PICAXE" side of your circuit where the FRM010 manual has this 1K resistor on the "FRM010" side of the circuit.


    This is the circuit for the serial input to pin 3 of the FRM010. It is not a serial download circuit so the resistor values from the FRM010 manual are the ones that will apply.
    I pointed out your second example in post 14, stating that as it stood, this (incorrect) configuration would act as a half "rail" voltage divider, or, in the case of an output to input, it would effectively halve the logic levels from the PICaxe to the FRM10. Hippy correctly pointed out that the datasheet states that your first example is the best configuration.
    Last edited by darb1972; 11-01-2018 at 19:35. Reason: Missed a word! Also couldn't spell this morning. Need coffee.
    Darb

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