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Topic: Please help me understand this partial schematic from Mega R3 (Read 1 time) previous topic - next topic

JoeN

1.  There are 6 lines going into the USB connector here.  Why?  USB connectors only have 4 lines.  What do the two lines on the bottom of the USB connector indicate?

2.  BLM21 seems to be a ferrite inductor.  But they are available in many different resistances.  How do I tell which one is appropriate for this part?

3.  What is this part?  I don't recognize it.  What does it mean?

Thank you.  I want to put together a clone of the Mega R3 to test out if I can mount BGA and QFN parts successfully.  This seems like it would be a nice challanging project.



I have only come here seeking knowledge. Things they would not teach me of in college.

Riva

1. The 2 extra wires are going to the USB shield.
2. Not sure.
3. Is a solder bridge so (in this case) you can bridge between grounds or keep them seperate.
http://forum.arduino.cc/index.php?action=unread;boards=5,67,10,11,66,12,15,17,21,22,23,24,25,29;ALL

JoeN


1. The 2 extra wires are going to the USB shield.
2. Not sure.
3. Is a solder bridge so (in this case) you can bridge between grounds or keep them seperate.


By shield do you mean the metal enclosure around the pins?  Thanks.
I have only come here seeking knowledge. Things they would not teach me of in college.

retrolefty



1. The 2 extra wires are going to the USB shield.
2. Not sure.
3. Is a solder bridge so (in this case) you can bridge between grounds or keep them seperate.


By shield do you mean the metal enclosure around the pins?  Thanks.


Yes.


JoeN

Is the metal enclosure for USB even an active part?  Is the circuitry for this just an attempt at lessening EMI?  Should I just leave that stuff off?  It saves on two parts that I don't own including the ferrite bead with an unknown value.
I have only come here seeking knowledge. Things they would not teach me of in college.

retrolefty


Is the metal enclosure for USB even an active part?  Is the circuitry for this just an attempt at lessening EMI?  Should I just leave that stuff off?  It saves on two parts that I don't own including the ferrite bead with an unknown value.


Sure, give it a shop. Commercial designs often have to apply for and pass FCC certification for EMI emission and rejection standards that a DIY may not have a problem with. I would assume you are not going to try and get FCC certification for your project?

Lefty

JoeN

#6
Feb 27, 2013, 08:45 pm Last Edit: Feb 27, 2013, 08:48 pm by JoeN Reason: 1


Is the metal enclosure for USB even an active part?  Is the circuitry for this just an attempt at lessening EMI?  Should I just leave that stuff off?  It saves on two parts that I don't own including the ferrite bead with an unknown value.


Sure, give it a shop. Commercial designs often have to apply for and pass FCC certification for EMI emission and rejection standards that a DIY may not have a problem with. I would assume you are not going to try and get FCC certification for your project?

Lefty


Nope, it's just a advanced soldering /custom PCB project because I need more experience with those things.  I'd like something useful if it works out is all.  An extra few Mega R3s compatibles would be great to have if I can make it work.  I'm going to leave off the power parts because I have a pretty decent bench supply and I am looking for other things I can trim off.  I see the PRC fuses are cheap so I will keep that, all the caps and resistors, of course, most of the SMD leds, and the two main chips and headers.  That USB grounding and regulator circuitry goes.  The opamp driven LED on PB7 goes too, I don't think that is worth enough to bother.  Actually, I am not even sure what that is the indicator for.  It's marked 'L' and is a yellow LED.

http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf
I have only come here seeking knowledge. Things they would not teach me of in college.

Zapro

I would have to say, if you have to ask about these kind of details, why do you want to mess with QFN and BGA packages ? If you want to lay out a board for such a complicated part, please learn basic electronics and schematics first. Just a small hint to get you started...

// Per.

JoeN


I would have to say, if you have to ask about these kind of details, why do you want to mess with QFN and BGA packages ? If you want to lay out a board for such a complicated part, please learn basic electronics and schematics first. Just a small hint to get you started...

// Per.


OK, since you are the schematic genius, what is the value of the BLM21 inductor?
I have only come here seeking knowledge. Things they would not teach me of in college.

Zapro

It's a well kept secret the Arduino folks haven't divulged anywhere. I've seen hundred of commercial designs where this part is omitted though...

It's not recommended to connect the USB shield to ground in the design, and should be avoided...

https://forum.sparkfun.com/viewtopic.php?f=14&t=22122

// Per.

68tjs

Quote
It's not recommended to connect the USB shield to ground in the design, and should be avoided.


It is recommended that the USB shield and the ground design has ONE common point and only ONE.

All shields must be connected to the ground in only ONE point to avoid circulation of parasistics current.
And, of course, ground must be a "good" ground : large diameter wire "star-connected" or wide area of copper on the printed circuit.

westfw

From the FTDI datasheet:
Quote
A ferrite bead is connected in series with the USB power supply to reduce EMI noise from the FT232R and associated circuitry being radiated down the USB cable to the USB host. The value of the Ferrite Bead depends on the total current drawn by the application. A suitable range of Ferrite Beads is available from Steward (www.steward.com), for example Steward Part # MI0805K400R-10.

This is associated with USB RFI compliance, so it won't change significantly for different USB chips...

retrolefty

Quote
The opamp driven LED on PB7 goes too, I don't think that is worth enough to bother.  Actually, I am not even sure what that is the indicator for.  It's marked 'L' and is a yellow LED.


That is the standard pin 13 LED made famous and standard to support the infamous Blink example sketch.

Lefty

JoeN


Quote
The opamp driven LED on PB7 goes too, I don't think that is worth enough to bother.  Actually, I am not even sure what that is the indicator for.  It's marked 'L' and is a yellow LED.


That is the standard pin 13 LED made famous and standard to support the infamous Blink example sketch.

Lefty


Oh, OK, it's the indicator indicator :).  I think I will leave that off.  I don't want to have to bother with an opamp just drive an 0805 LED on the board since I don't need the opamp for the power circuitry that I am going to leave off too.
I have only come here seeking knowledge. Things they would not teach me of in college.

CrossRoads

So drive the LED directly, like the Duemilanove and early Uno's did, before the op amp buffer was used.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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