bfinio
Posts: 38
Joined: Mon Oct 14, 2013 8:53 pm

Why isn't 1200mA USB output enabled by default?

Mon Aug 11, 2014 2:09 pm

I have been searching for an "official" answer to this. At first it seems odd to me to increase the total power draw capability to 2A, but only make 600mA available to the USB ports. The closest explanation I've found so far is from a forum moderator in this thread:
If anyone has a B+ board, there is a hidden config option:

Code: Select all
safe_mode_gpio=4

By default the USB ports are limited to 600mA.
With the new option, this is increased to 1200mA.

Note: this was just for internal testing. In the next firmware update this will be renamed to

Code: Select all
max_usb_current=1

The idea is you only set this option if you know you have a good (e.g. 2A) power supply.
Googling also brings up this post, Testing & Setting the USB current limiter on the Raspberry Pi B+, which suggests Warning: DO NOT do this on a whim. Only do this if you are absolutely sure that you need to do it!

I guess I understand the reasoning that many people might still just have 1A power supplies, so you need to manually enable the 1200mA output if and only if you know you have a higher-rated supply. However, I can also see new users being confused when they read that the B+ can "handle more USB devices" or "draw 2 amps" and then get confused when they need to manually allow their USB ports to actually supply a lot of current, since they expect that behavior to work out-of-the-box.

Anyone have an official explanation for this either way?

ShiftPlusOne
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 6053
Joined: Fri Jul 29, 2011 5:36 pm
Location: The unfashionable end of the western spiral arm of the Galaxy

Re: Why isn't 1200mA USB output enabled by default?

Mon Aug 11, 2014 2:19 pm

Don't read anything I say as 'official', but it's because many people have terrible power supplies. When you have a 1A supply and the usb devices draw a lot of current, you can brown out the supply and cause the pi to reset or hang. If the usb current is limited, it's more likely to only take out usb devices and the pi can recover, which is more desirable. Since good 2A supplies are not the norm, it makes more sense to leave the 1.2A option for people who know what they're doing.

No matter which default you go with, someone will complain anyway.

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