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Topic: Two questions on simple PCB schematic (Read 672 times) previous topic - next topic

ardilla

This is my second attempt at creating a PCB and have a couple of questions on my schematic. Basically this is a bare Arduino setup with two LEDs, an RF receiver, and some pins to program it with my FTDI cable. Nothing more than that; I'm trying to keep this attempt as simple as possible.

1) The top right part shows a voltage converter and decoupling capacitors according to this breadboard Arduino article. My prototype works fine so I know I assembled this correctly on my breadboard but I'm not sure if I drew this properly in my schematic.

2) The "S2" push button on the top left (here's the actual one I'm using) has two legs that aren't connected to anything. Should I connect 1-2, and 3-4 together or are they ok the way they are?

By the way, if you happen to spot anything that's not properly done (even if it is cosmetic) I'd appreciate a comment. I'd like to learn as much as I can from this process.

Cheers


Nick Gammon

#1
Sep 25, 2013, 08:42 am Last Edit: Sep 25, 2013, 08:44 am by Nick Gammon Reason: 1
You should have 100 nF decoupling capacitors on VCC and AVCC.

See: http://www.gammon.com.au/breadboard
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dc42

Also:

1. You should not have Aref connected to Vcc. Leave Aref unconnected, or connect a 0.1uF capacitor between it and ground.

2. Consider adding a 6-pin ICSP header, so that you also have the option to program it and/or change the fuses via ICSP.
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Marvin Martian

Cosmetically, I'd say keep the schematic connections horizontal or veritcal... And make sure you add a 'pip' at every junction (an ERC will pick missing ones up)

MarkT

Definitely make all supply symbols up (if positive) or down (gnd / negative).  That's not cosmetic, there is no
such thing as a sideways ground / supply symbol.

Are you working on a board layout?  If so I'll say groundplane...
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ardilla

Thanks everyone. I will take your advice and improve my design.

@dc42: I looked at the datasheet for the ATMega328 regarding your AREF capacitor suggestion and you are definitely right. I'm wondering why the "official" Arduino page hooks it up to VCC but I'm going to go with what the datasheet (and you) say.

Quote
Cosmetically, I'd say keep the schematic connections horizontal or vertical

I tried removing the diagonals and it makes the diagram a lot less readable. Are diagonals one of those things that are frowned upon by professionals or is there some other reason behind it?

By the way, nobody mentioned anything regarding the momentary switch or the voltage regulator so I'm assuming I drew these correctly (there is always a first time!).

James C4S


I'm wondering why the "official" Arduino page hooks it up to VCC

What page?  The schematics show AREF going to an empty pin header with a 100nF cap to ground.
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ardilla



I'm wondering why the "official" Arduino page hooks it up to VCC

What page?  The schematics show AREF going to an empty pin header with a 100nF cap to ground.


The breadboard Arduino link on my first post. I said "official" (between quotes) because I understand that this particular page is a wiki. If the datasheet says one thing and this page hasn't been corrected by anyone leads me to think that there is some reason behind the AREF --> VCC hookup. Plus it does work, as I have done many breadboard Arduinos using these instructions (haven't had a chance to test it using a complex circuit though). Sadly my EE knowledge is limited so I can't really know why they did this, that's why I'm going to be following the datasheet's advice.

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dc42




I'm wondering why the "official" Arduino page hooks it up to VCC

What page?  The schematics show AREF going to an empty pin header with a 100nF cap to ground.


The breadboard Arduino link on my first post. I said "official" (between quotes) because I understand that this particular page is a wiki. If the datasheet says one thing and this page hasn't been corrected by anyone leads me to think that there is some reason behind the AREF --> VCC hookup. Plus it does work, as I have done many breadboard Arduinos using these instructions (haven't had a chance to test it using a complex circuit though). Sadly my EE knowledge is limited so I can't really know why they did this, that's why I'm going to be following the datasheet's advice.


There are other things wrong with that page. For example, at least one 0.1uF ceramic capacitor or similar should be connected right across the chip Vcc and ground pins.
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SirNickity

What you do with Aref depends on how you set up the hardware.  The function of that pin is determined by what reference you're using for analog readings -- it can be a couple internal references (in which case, you only want the 0.1uF cap to ground to filter noise), or you might be providing a reference voltage on that pin (e.g., connecting to Vcc.)  You can ruin the chip by setting it for internal reference and then tying the pin to Vcc, so the safest option is to use only the cap.  Like the Arduino, you can break it out to a pin header for more flexibility.

Lines in your schematic can be diagonal if it helps readability.  Convention is straight lines and 90-deg turns (unlike PCB design, which prefers 45s).  Why?  It just is.  No great reason I'm aware of, but sticking to typical design patterns helps others read it more easily.

On that note, inputs on left (including power), outputs on right, signals flowing left to right where possible.  You won't be shot for violating these rules, but unjustified diversions from this will confuse and frustrate your readers.

On the regulator, a larger cap on the input would be nice -- say 100uF.  A diode from the regulator's output TO the input will bleed the regulator cap into the power input if the output somehow ends up higher voltage.  This is good practice but not essential if you can assure this never happens.

More decoupling caps has been covered, but +1 anyway.

No problem on the unconnected switch terminals.  If it's a single-pole switch, they're connected internally already.  If not, you don't need to switch the circuit twice, although doing so could in theory reduce the current load on the contacts.  In this application it's probably not much help.

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