Lets look at a circuit diagram this time!!

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#1
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Hi Guys, I am working on a project involving mini usb connection. I know this is probably the wrong place to post such question, but be a good embedded engineer, I suppose we still need to know some basic stuff about circuit diagram.

So here is the Circuit diagram:
[img]
By gpakillsme at 2012-08-09[/img]

Few places I dont understand, hope anyone could answer it.
1. for VBUS why there are two caps there?
2. between GPIO and mini usb connector, both USB_VBUS,PA11,PA12 have a Resistor in between, what's the reason to put a R there?
3. Last question, sounds stupid. Based on description, the USB_PWR is used to power external usb device, why putting a bipolar transistor there?

Really appreciate your answer!

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1. Low frequency and high frequency bypass.
2. the 22r is series termination for possible reflections. the top resistor is 0R.
3. transistor is for turning the device power on and off. There is no current limiting.

Also note there is no ferrite beads for RF supression and no esd protection. Rather than the transistor, there are specific ics that will do the current limit and provide an error signal.

Why did you post in the AVR Forum, this sounds more like General Electronics.

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Kartman wrote:
1. Low frequency and high frequency bypass.
2. the 22r is series termination for possible reflections. the top resistor is 0R.
3. transistor is for turning the device power on and off. There is no current limiting.

Also note there is no ferrite beads for RF supression and no esd protection. Rather than the transistor, there are specific ics that will do the current limit and provide an error signal.

Why did you post in the AVR Forum, this sounds more like General Electronics.

thank you! :D

Hope you dont mind I ask you a little bit more, for #2, how a resistor can stop reflection. you dont have to answer me if the answer is way to long. please let me know what should i search in google.

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first, thanks for you reply...so the impedance mismatch will cause the reflection, but why adding a resistor can over come this issue?

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shaolei wrote:

first, thanks for you reply...so the impedance mismatch will cause the reflection, but why adding a resistor can over come this issue?

Because it matches the impedance, so there is no reflection?

Google "USB series resistor", I have two good application note PDFs at top results..