[PLUG] Powered USB-A hub - LSUSB MaxPower

[Galen Seitz]

On 1/17/23 19:15, MC_Sequoia wrote:
> You want to a guess what the MaxPower is listed for the Logitech V20 portable usb speakers are that are plugged into my pc?
> 
> I'll give you one guess.
> 
> Yep, MaxPower is listed as 590 mA
> 
> "The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices.
> 
> A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0."
> 
> "Bus 003 Device 002: ID 046d:0a04 Logitech, Inc. V20 portable speakers (USB powered)
> Device Descriptor:
>    bLength                18
>    bDescriptorType         1
>    bcdUSB               1.10
>    bDeviceClass            0
>    bDeviceSubClass         0
>    bDeviceProtocol         0
>    bMaxPacketSize0         8
>    idVendor           0x046d Logitech, Inc.
>    idProduct          0x0a04 V20 portable speakers (USB powered)
>    bcdDevice            0.07
>    iManufacturer           1 Logitech
>    iProduct                2 Logitech USB Speaker
>    iSerial                 0
>    bNumConfigurations      1
>    Configuration Descriptor:
>      bLength                 9
>      bDescriptorType         2
>      wTotalLength       0x00e9
>      bNumInterfaces          3
>      bConfigurationValue     1
>      iConfiguration          3 G6 2005/03/29 10:05
>      bmAttributes         0xc0
>        Self Powered
>      MaxPower              500mA
> 
> 
> The total RMS Power of the Logitech V20 Portable USB Speakers = 2 W or 2000 mA.

power = volts * amps
With VUSB being 5V, this gives
amps = power / volts = 2W / 5V = 400 mA

I don't know what IC(s) is used in these speakers, but this is 
consistent with a system (USB DAC + amp) that is 80% efficient.

500 mA * 80% = 400 mA

> 
> Whether I'm driving audio through the speakers our not, the MaxPower stays at 500 mA.
> 
> LSUSB MaxPower is the power output of the USB port the USB device is connected to and not what that MaxPower of the device is or can take.
> 
> All the USB ports on my pc are 2.0, if you've a mix of USB ports or different PCs with different USB port specs, you can prove this by moving the USB device to the different ports and checking the MaxPower that LSUSB lists.

MaxPower is set in the USB device descriptors.  It is independent of the 
host.  Here's an example where I actually changed the MaxPower number of 
an FTDI USB to serial adapter (FT232R).

# Before touching the MaxPower descriptor.
lsusb -v -d 0403:6001 | egrep 'iProduct|MaxPower'
   iProduct                2 FT232R USB UART
     MaxPower               90mA

# Using W10 and the FTDI tool named FT_PROG, I reduced MaxPower.
lsusb -v -d 0403:6001 | egrep 'iProduct|MaxPower'
   iProduct                2 FT232R USB UART
     MaxPower               84mA

Here's a blog post that does a decent job of explaining MaxPower.
<http://dangerousprototypes.com/docs/Designing_USB_Devices_for_proper_current_and_MaxPower>

Here's the datasheet for the FT232R.  See sections 6 and 8.
<https://www.ftdichip.com/old2020/Support/Documents/DataSheets/ICs/DS_FT232R.pdf>


galen
-- 
Galen Seitz
galens at seitzassoc.com