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

On 2021-Dec-19, at 09:11, Mark Millard <marklmi@yahoo.com> wrote:

> On 2021-Dec-19, at 08:18, bob prohaska <fbsd@www.zefox.net> 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. 
>> 
>> There's a table at
>> https://hddfaqs.com/seagate-st1000lm048/
>> listing power requirements for the drive:
>> Required Power For Spinup: 1000 mA
> 
> (I'm guessing they list that as the largest surge current.
> But they do not list figures for READ or WRITE activity.)
> 
> https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#typical-power-requirements
> 
> lists the "Maximum total USB peripheral current draw" as: 1.2A
> (so 1200 mA) for the B+, 2B, 3B, 3B+, 4B, and Pi400. The 3B and
> 3B+ list a "Recommended PSU current capacity" of 2.5A, the 4B
> lists 3.0A.
> 
> Some keyboards or other such could lead to problems if also
> connected at power up: Having both a keyboard and mouse at
> power up, in addition to the drive, could be a problem:
> 
> QUOTE
> keyboards and mice can take as little as 100mA or as much as 1000mA
> END QUOTE
> 
> So stick to a low total power for your other USB devices that
> are to be already connected at power up. You might have to
> carefully pick what keyboards/mice/whatever to fit the 200mA
> budget that is left --or plug some things in only after the
> drive has spun up.

There is a separate issue of the USB standards for maximum power
for one USB2 port. Quoting:

https://resources.pcb.cadence.com/blog/2020-what-are-the-maximum-power-output-and-data-transfer-rates-for-the-usb-standards

QUOTE
In general, the specifications for a USB 1.0 and 2.0 standard
downstream port, delivers up to 500 mA or 0.5A.
END QUOTE

This means that your drive requires more current (power) than
one USB2 port has as a maximum.

Using an adapter that gets power from 2 USB2 ports instead of
just one would be appropriate if what you are using gets power
from only one USB2 port.

FYI: USB3.0 provides up to 900mA (0.9A, 4.5W) for non-charging
ports (still insufficient) and 1500mA (1.5A, 7.5W) for special
charging ports (dedicated to charging or charging downstream).


>> Power Required (Seek): 1.7 W
>> Power Required (Idle): 1.6 W
>> Power Required (Standby): 0.18 W
>> 
>> So far I haven't tried to power cycle the combo, that might not work. 
>> Still, it's been an informative exercise. Getting rid of the hub is a
>> welcome simplification. The machine is still up after standing overnight.




===
Mark Millard
marklmi at yahoo.com