Re: Dealing with slow USB disks, was: Re: Saving environment variables in u-boot

On 20/12/2021 9:55 am, Mark Millard wrote:
> On 2021-Dec-19, at 13:39, MJ <[email protected]> wrote:
> 
>> On 20/12/2021 3:18 am, bob prohaska wrote:
>>> On Sun, Dec 19, 2021 at 10:03:47PM +1100, MJ wrote:
>>>>
>>>> I would think a mechanical USB is going to pull a "lot" of power when beginning spin-up, but once rotating should be easily powered by a USB hub. Though this would not explain how it works on RPI4 unless the powered hub you're using is USB2.
>>>>
>>> That's what I thought too. I certainly didn't expect the disk to work
>>> without a powered hub. The Pi4 is a different animal; it has USB3 ports
>>> and more power available. That the mechanical disk works at all on the
>>> Pi3's USB2 ports without assistance is quite surprising.
>>
>> See here: https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#typical-power-requirements
>> It mentions 1.2A, which covers your load, but, I would suspect if you attached, for example, a USB 'thumb' drive or other devices you would cause problems.
> 
> Unfortunately there is more involved: USB2's standard
> indicates 500mA (0.5A) at maximum on 1 standard USB2
> port. It takes 2 USB2 ports to get to a total of 1000mA
> (1A) (unless a port is designed to go beyond the
> standard). To my knowledge most RPi*'s are not designed
> to support more than the standard USB2 power on any of
> its USB2 ports. (The 3A+, Zero W/WH, and Zero are
> apparently exceptions, depending on the power supply
> used and such.)


You are correct, however, it seems the RPI foundation violates the "standard" and creates their own for Bs:

https://raspberrypi.stackexchange.com/questions/51615/raspberry-pi-power-limitations
(Section: How much current can be drawn from the USB ports?)

M