Errors in my Schematic?

Discussion in 'Homework Help' started by urb-nurd, Apr 9, 2015.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    I am submitting this schematic with a piece of school work in the near future.

    I was wanting to have a second set of eyes make sure there is nothing erroneous in the layout if possible.

    The circuit is a data logger for recording direction and acceleration of a car.

    Here it is
    schematic.PNG

    Thanks folks!
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Looks pretty good.
    You have a few junctions that aren't dotted:

    upload_2015-4-9_16-3-21.png upload_2015-4-9_16-4-29.png upload_2015-4-9_16-5-55.png
    And this has an unnecessary bend:
    upload_2015-4-9_16-5-21.png

    You do not show a voltage for Vdd to the MCU. Of course, the devil is in the details. Does your program work? What are you outputting?

    John
     
  3. WBahn

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    Mar 31, 2012
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    The Vdd value is shown at the output of the regulator. It is good practice to only show this value in one place (or, more specifically, to only have it defined in one place -- if you can define it in one place and have that value be used to show the value in multiple places, that's wonderful -- but few schematic capture packages allow that).
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    that pic has internal osc upto. 8mhz, no need for the Xtal.
     
  5. WBahn

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    Mar 31, 2012
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    Unless you want an accurate 8Mhz clock.

    Having said that, they are calibrated at the factory so they are much better than the internal oscillators of old. I don't know what their specs are.
     
  6. jpanhalt

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    Jan 18, 2008
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    From Eagle 7.2.0

    upload_2015-4-9_18-6-30.png

    I will send Cadsoft-USA your advice. I think Cadsoft will ignore it. It has been that way for more than 10 years.

    John
     
  7. WBahn

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    Mar 31, 2012
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    I'm sure they will ignore it. That doesn't change anything regarding what I said. Software vendors routinely follow bad practices and are hesitant to correct them because of the significant maintenance effort involved in doing so. Just because Vendor X has been doing it that way for Y years doesn't mean that it is good practice.

    It is bad practice to manually label something in multiple places. What happens when you adapt the design and change the logic from 5V to 3.3V? Now you have to go through all of the schematics making sure that you change all of the labels. What are the odds that at least one label isn't going to get changed? If you have a CAD package that supports defining global attributes, then by all means do so, but if it doesn't, then you are setting yourself up for lots of problems if you define something in umpteen different places.

    If you encode the value as part of the global label, such as naming the node Vdd5 instead of just Vdd or putting the voltage value as a label in the symbol, then when you adapt the schematics you can change the global label definition from Vdd5 to Vdd3 or change the label in the symbol and all of the schematic renderings will be updated. That is 100% in agreement with what I said.
     
  8. jpanhalt

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    First, in Eagle, one does not "manually" label the supply. It just so happens that the device chosen for 3.3V and 5 V have the same appearance on the schematic, except for the label (name).

    You are assuming the supply to the MCU is also 3.3 V. Can you cite an authority, such as a standard, for the practice you suggest?



    John
     
  9. WBahn

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    The output of the regulator is connected to a symbol labeled Vdd.

    The supply pin of the MCU is connected to an identical symbol labeled Vdd.

    If two identical symbols that are both labeled the same thing are not the same node, then that is really poor.
     
  10. jpanhalt

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    But can you cite a standard that supports your conclusion?

    John
     
  11. WBahn

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    No, I can't. I can probably dig up several places that emphasize that over-defined systems and documentation represent poor practice, precisely for the reason I gave (as well as several others).

    Can you cite a standard that supports YOUR position that a specific voltage has to be shown everywhere that Vdd appears?
     
  12. jpanhalt

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    Without some standard, you might have been more accurate calling it your opinion.

    I do think that the TS might be advised to label each supply as either just Vdd or with the voltage. I prefer the voltage, as that is unambiguous and is what Eagle default uses. Eagle will catch you in ERC, if you use Vdd for two different voltages.

    John
     
  13. kubeek

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    Sep 20, 2005
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    As far as I remeber from past experience with eagle, whatever you name that voltage connection is the name of the net it is connected to. Easy and simple, especially whe you have more than one supply rail, and of course nothing prevents you from using different symbols for different supply levels.
     
  14. WBahn

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    Just where did I say that it was required or that it was per some standard? I said that it is "good practice" to only define things in one place, and it is.

    Did you label it as YOUR opinion when you critiqued his schematic for not showing the specific voltage value for Vdd at the MCU, despite the voltage associated with the node Vdd being documented at the regulator?
     
  15. RichardO

    Well-Known Member

    May 4, 2013
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    What is your reason for the 2.2k and 3.3k resistors on the SD card pins?
     
  16. WBahn

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    That's a good question.

    My guess is that it is a 5V to 3.3V voltage divider (which he almost certainly does NOT want).
     
  17. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    I copied those resistors from an example schematic, i didn't really look into the purpose as it was the last thing i did before giong to bed for the night.

    Heres where i copied it from.
    sdcard.jpeg

    I didn't know why they were present, but added them just before shut down after seeing them on a few schematics.

    Ill read through the comments in a moment and reply properly.

    mornings are torture.
     
    Last edited: Apr 10, 2015
  18. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    It just occurred to me that the resistors could be to allows a MCU with a 5V supply to interface the 3.3V sd card. Which is what WBahn was saying i think.
    If the MCU is powered with 3.3v (mine is to allow one regulator to power all components for ease) then i guess the resistors are un needed as the High voltage on the pins is 3.3V due to the supply to the UC being 3.3V .

    Just a guess
     
  19. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
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    [​IMG]

    Given i am using a 3.3V supply to my MCU, then i can omit the resistors.

    I have also updated the schematic to remove the erroneous connections, though i am still unsure the concensus of my labelling of VDD and voltages haha.
     
  20. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
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    Although, i think i will look into changing my supply regulator for a switching one.
    That way i can power the device with one 3.7V cell.
    The gps module i have chosen requires at least 4V, so i will need to be using both 5 and 3.3V for this it seems.
    I guess i will change the output of my supply circuit to 5V and then use the resistors to supply 3.3V to the SD card.

    if i look at the power requirements of the device, maybe i can get away with a 1.5V cell and a boost converter.

    We shall see
     
    Last edited: Apr 10, 2015
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