kahlenberg
Posts: 41
Joined: Mon Jul 29, 2013 10:47 am

Current limits of USB ports on RPi3

Mon Nov 21, 2016 10:13 pm

Hello,

Is there any current limits of USB ports on Raspberry Pi 3? If yes, which chip is responsible for that? There are 4 USB ports, If each port can give maximum 500mA, it makes 2A for all 4 ports. RPi itself needs also app. 1A, so a 3A power supply must be used. What happens If I use 2A power supply?

I didn't find any schematic for RPi3. Only reduced schematic. It does not show where the power lines of the usb ports are connected.

I am asking because, I attached a DVB-T dongle directly to one of the USB ports. I scan for channels using tvheadend software, but there are lots of error messages saying "no scan data" or similar (DVB-T Stick is recognized). I am suspecting that DVB-T dongle needs more power than USB limit. Before I buy a powered USB Hub, I want to be sure that RPi limits the current.

Thanks.

fruitoftheloom
Posts: 21345
Joined: Tue Mar 25, 2014 12:40 pm
Location: Delightful Dorset

Re: Current limits of USB ports on RPi3

Mon Nov 21, 2016 10:20 pm

kahlenberg wrote:Hello,

Is there any current limits of USB ports on Raspberry Pi 3? If yes, which chip is responsible for that? There are 4 USB ports, If each port can give maximum 500mA, it makes 2A for all 4 ports. RPi itself needs also app. 1A, so a 3A power supply must be used. What happens If I use 2A power supply?

I didn't find any schematic for RPi3. Only reduced schematic. It does not show where the power lines of the usb ports are connected.

I am asking because, I attached a DVB-T dongle directly to one of the USB ports. I scan for channels using tvheadend software, but there are lots of error messages saying "no scan data" or similar (DVB-T Stick is recognized). I am suspecting that DVB-T dongle needs more power than USB limit. Before I buy a powered USB Hub, I want to be sure that RPi limits the current.

Thanks.
The RPi 3B uses a LAN9514 Chipset on the Single USB Bus to provide 4 x USB and the Ethernet Port:

http://www.microchip.com/wwwproducts/en/LAN9514


1200mA is USB total, not individually, see:

https://www.raspberrypi.org/help/faqs/#powerReqs
Retired disgracefully.....
......to a more gentile life !

gregeric
Posts: 1509
Joined: Mon Nov 28, 2011 10:08 am

Re: Current limits of USB ports on RPi3

Mon Nov 21, 2016 10:44 pm

The Pi3 uses an AP2553 to limit the current on the four downstream USB ports to a total of around 1200mA. The four USB ports' 5V lines are wired together, so total current is limited to that 1200mA figure before that switch cuts the supply. A diagram of that portion of the schematic is shown here: viewtopic.php?f=63&t=138280&hilit=+schematics#p917849

With your 2A PSU & no cameras or other peripherals attached, you should still be able to draw the full 1200mA from USB device(s).

Do you see any messages in the kernel log when you are scanning in tvheadend? (Open a terminal & type dmesg to see the log).

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