power raspberry pi from usb port?


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3 years 2 months ago #3595 by reverendalc
i've read all sorts of things on the internet. standard usb is 500ma.
some devices will take more than 500ma, and some motherboards will give more than 500ma.

i know apple computers will allow up to 1000ma on usb when a special handshake occurs between the mac and an apple brand accessory like the usb superdrive.

i know hackintosh builds like my own (with an asus mobo) will indiscriminately deliver up to 1000ma via usb

i would like to power the rbp from an mba. it definitely has enough power to boot up, but does anybody have any experience with the power requirements of the EFI flash process and the 500ma supply?

i'd love to hear stories of wins or losses

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3 years 2 months ago #3638 by thaGH05T
Well, you have to take into account the power requirements of the pi and the chip it is trying to communicate with as well as the voltage the SPI chip is going to need. In my experience you will need 2.5 amps on Pi 2 to successfully initiate an SPI transaction. In my opinion, this will not work for what you are trying to do. Although, since you are trying to create a tool with a screen etc... I suggest looking into 18650 batteries to power the pi as a stand alone device. Assuming this is going along with the conversation we had the other day.

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3 years 2 months ago #3643 by reverendalc
maybe in a perfect world? perhaps that's why my bus pirate won't initialize.

i exclusively use my iphone6 power brick, which is rated at 1a, and I have countless successful flashes under my belt.

i'd be curious to know what the lowest amperage power supply anybody has used successfully and under what conditions

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3 years 2 months ago #3645 by thaGH05T
You can use the lowest rated for your model pi, but you will need to use short wares to account for signAL loss and voltage drop.

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3 years 2 months ago - 3 years 2 months ago #3648 by reverendalc
i've considered the battery route to go completely portable, in which case a screen would be super fun and power wouldn't be a big deal with the right battery.

i'm currently using 6" leads, and VNC from a macbook air for control and hex edit. that's why i'd like to power it from the MBA.

else if

there's a really cool screen for the rbp but it uses some of the SPI pins needed for flashing. is it possible to reprogram/repurpose the GPIO header? the rbp screen that uses the display connector is friggin huge and shits all over my tiny portable plans

FWIW: i'm using rbp2 model b
Last edit: 3 years 2 months ago by reverendalc. Reason: FWIW

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3 years 2 months ago #3717 by thaGH05T
there should be multiple pins for SPI alternates. Post what model pi you have and what screen you wanna use and I will look into it.

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3 years 2 months ago #3727 by reverendalc

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3 years 2 months ago #3743 by thaGH05T
That TFT display is SPI driven, so you cannot use it. You need to find a display that does not use the UART/SPI protocol.

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3 years 2 months ago #3745 by reverendalc

thaGH05T wrote: there should be multiple pins for SPI alternates. Post what model pi you have and what screen you wanna use and I will look into it.


that's what my understanding was, but you were all instilling hope and such

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3 years 2 months ago #3746 by thaGH05T
There has to be a display out there that uses a different protocol, I have not looked around yet, but I imagine it isn't too far fetched to say you could do both. You could use the display port with a custom solution?

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3 years 2 months ago #3774 by reverendalc
there's a great rbp touchscreen which uses the display port exclusively, but it's freaking huge.

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3 years 2 months ago #3779 by thaGH05T
You talking about the 7" touch that uses the ZIF?

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3 years 2 months ago #3784 by reverendalc

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3 years 2 months ago #3799 by thaGH05T
There are plenty of screens out there, but the issue is the drivers for them. I have worked with a few that i liked, but they were all SPI based.

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3 years 2 months ago #3801 by reverendalc
That's exactly what I'm running into. Maybe I'll just tape two pis back to back, and vnc the one with the display into the other one... LOL

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