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I'm building a mechanical keyboard and I need more current from my PC than I can get from one usb port. Can buy/make a usb splitter that I can plug into two A ports on my PC and plug into my USB-C MCU? The MCU in question can handle up to 2A and I don't need that much...but I do need more than 900mA.

Am I better off designing my keyboard with a second usb port for additional power? (In which case I'll need to ask another question to understand how!)

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  • \$\begingroup\$ why does your keyboard need 2 amps? \$\endgroup\$
    – user253751
    Apr 12, 2022 at 15:17
  • \$\begingroup\$ Led strip. Even at half brightness, all the LEDs together will exceed 900mA, let alone the current the MCU itself needs. I probably won't need 2A...but definitely more than .9A \$\endgroup\$
    – AerosolSP
    Apr 12, 2022 at 15:24
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    \$\begingroup\$ did you test the brightness? 10W of LED light is quite a lot \$\endgroup\$
    – user253751
    Apr 12, 2022 at 15:36
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    \$\begingroup\$ Sounds like your LED strip choice is not quite appropriate for the application, unless you want to fully illuminate the whole room. \$\endgroup\$
    – Eugene Sh.
    Apr 12, 2022 at 15:38
  • \$\begingroup\$ Honestly, I have not! They've only arrived today and I haven't gotten home from work yet. But I can't wait until after I've put everything together before finding out that I have to turn the brightness down lower than I'd like in order to not trip a fuse. So I want to make atleast another 500mA available now so I have some wiggle room. \$\endgroup\$
    – AerosolSP
    Apr 12, 2022 at 15:42

1 Answer 1

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Design your project with USB C and a C to C cable in mind.

Eve though USB A ports on a desktop only provide about 900ma, the USB C ports on a desktop machine typically provide higher currents, up to 3A.

On your board, measure the voltage on both CC pins using a 10K resistor and then to your microcontrollers ADC, and take the highest voltage of the 2 measurements.

  • If you measure above 1.31V, the downstream facing device advertises maximum 3A, meaning it is good enough for all your device features

  • If you measure above 0.70V, the downstream facing device advertises maximum 1.5A, so you limit the brightness/max PWM in software to stay under the limit

  • If you are communicating USB 3.2 dual lane to the other device, your limit is 1.5A

  • If you are communicating USB 3.2 single lane to the other device, your limit is 0.9A

  • If you are communicating USB 2.0 to the other device, your limit is 0.5A

  • Otherwise, the host must be in suspend, so the limit is 0.1A

By gracefully reducing the maximum consumption based on the advertised values, you increase compatibility with other devices

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  • \$\begingroup\$ Thanks! Just one thing: my desktop machine has no USB C ports, so I'm ultimately limited anyway! \$\endgroup\$
    – AerosolSP
    Apr 19, 2022 at 10:07

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