Hi. I’m new to KiCAD and I want to design a circuit similar to the Arduino Uno but with the use of a USB mini B connector instead. In the KiCAD library, the schematic found has 6 pins (Vcc, Gnd, D+, D-, ID and Shield). What I want to know is if i should directly connect the Shield pin to ground or leave it unconnected. Thanks
There is no simple answer. It depends on you as a designer - what you wont to achieve. Many years ago I spend thinking how to do it about two months (during making the project). At the end everything else was done and I have to decide something so I did it.
My problem was the very small plastic case made of two halves. At the lines of connecting those halves the ESD could go inside my device and I couldn’t made my PCB smaller enough to make case-pcb distance enough big to protect against ESD (about 15mm distance is enough). After many times changing my decision I end up with Shield not directly connected to GND, but as I am not sure if my end decision was the best one I will not describe my solution in details.
I do not have much experience with USB, but my guess is you’d best connect it to GND. Reasoning is that this metal is likely the first contact between USB devices and connection to GND will discharge static charges the furthest away from electronics.
A simple approach is to just measure a few USB devices, and in 4 out of 5 the shield seems to be connected to GND.
Same on an old PC motherboard, all shielding, and even the annular rings of the mounting holes are directly connected to GND.
About the ID pin…
I think you should also connect it to GND. As far as I know it’s used to switch between host and device on USB OTG, but as said before, my knowledge of USB is very limited.
Varistors on the data lines?
It could be, but it’s much more common to see TVS diodes.
There are special devices being made for this with TVS diodes both to GND and to Vcc for each data line and also a TVS diode between Vcc and GND. For example:
In this symbol 2 is GND and 5 is Vcc, and there is room for 2 signal lines.
One signal line goes from 1 to 6 through the IC, the other from 3 to 4.
Do not route your signal from the connector to your IC with a T-branch to the protection circuit. ESD events have very steep flanks, and therefore lots ofhigh frequency content and the physical layout of the PCB tracks is very important.
Thanks for the info on the TVS diode. I’d do more research on the shielding part.
I’ve for a few Arduinos (Uno, Mega, Leonardo). And all of them have their USB shields connected to ground so you should be safe to follow this approach.
How furthest away if connected? This is a dividing the hair into four but if there is some charge at your GND then if GND is connected with shield and shield got the first contact then the charge goes out of your GND as fast as possible. But if you connect shield to GND by ferryte bead then (I think) everything happens the same way except the dI/dt is smaller so I think better.
I’m not sure of my opinion here.