Hello everyone, I have a several questions regarding the arduino uno r3 schematic. It's okay if you cant answer all of them.
(1) There are these new symbols I've never seen, I think they are solder bridges. One is named "ground" and one is "reset". One is shown between the UGND line and the GND line. What the heck is that for? And what is the reset solder bridge for?
(2) Also there are two resistors way out in the right space named RN3B and RNC3. What are those resistors for and where are they supposed to be connected?
(3) Why don't they just use a 500 ohm resistor instead of two 1k resistors in parallel for the "power ON" green LED?
(4) Can I delete the buffer and yellow led circuit for pin 13?
(5) There appears to be a comparator that detects when half of the input voltage is below 3.3V and then switches the power over to the USB power if it is plugged in. Can i delete the comparator circuit if I plan to just run the circuit off purely USB power when I am programming it?
(1). The solder bridges make it easy to join the circuit if required.
The GND allows the USB GND to be connected to the main GND.
I read somewhere that it may be desired to cut the RESET trace under some circumstances. The pads make it easier to restore the cut.
(2). These are 2 resistors in a resistor pack that are on the board but not connected.
Look at the USB input and you will see 2 x 22 ohm resistors RN3A and RN3D. These are two resistors in the pack that are used. Note the resistance is 22ohms on all resistors. The number at the end of each is the PIN number in the pack.
(3). A 1k resistor pack on the board, RN4 only uses two of the resistors so the other two are used here rather than installing another R and wasting these two.
(4),(5). Why would you want/need to modify the board???
Thanks for the great response, it's all starting to make sense! I have just a few questions about what you said.
weedpharma:
(1). The solder bridges make it easy to join the circuit if required.
The GND allows the USB GND to be connected to the main GND.
I read somewhere that it may be desired to cut the RESET trace under some circumstances. The pads make it easier to restore the cut.
Why would i want USB ground to be connected to main ground? According the to schematic, it looks like they need to be separate, but don't have to be..
Finally it makes sense why the reset bridge is in there!
(2). These are 2 resistors in a resistor pack that are on the board but not connected.
Look at the USB input and you will see 2 x 22 ohm resistors RN3A and RN3D. These are two resistors in the pack that are used. Note the resistance is 22ohms on all resistors. The number at the end of each is the PIN number in the pack.
Ok so is there ever a scenario where I would want to use the other resistors on the USB input or can I just leave that alone?
(4),(5). Why would you want/need to modify the board???
I am fabricating a clone, so i do not want unnecessary features installed. Thanks Grumpy Mike!
RonTheSwanson:
I am fabricating a clone, so i do not want unnecessary features installed.
You might be surprised as to what turns out to be "necessary".
The power switching circuitry is there for a reason.
There are very few circumstances indeed that you do not want USB ground to be connected to the main ground. This would only happen when some other circuitry is connected which also connects the main ground to another ground point on the PC and there is some interference problem. In all other cases, this connection is essential.
Using resistor arrays saves space and generally, cost, particularly in bulk production. Fewer components to pick and place.
Using resistor arrays saves space and generally, cost, particularly in bulk production.
Yes that is what the sales man was always telling us, but in practice this turned out not to be so. The cost bit especially they were more expensive than four resistors and did not save anything on the assembly bill.
Noting the rock-bottom price of single SMD resistors, I am not surprised and was a bit tongue-in-cheek when I posted that. And it is not by any means always convenient - or good design - to route to place the resistors in one package - though at other times it is a very good idea.
Which poses the question - just why did the Arduino team choose them?