Author Topic: Tablet charging in host mode  (Read 1800 times)

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Offline gaminnTopic starter

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Tablet charging in host mode
« on: July 30, 2019, 07:56:11 pm »
Hi,
I'm looking for +- 8" 1280 x 800 low end cheap tablet that can charge itself while in USB host mode. I need to connect USB keyboard (or another USB device) to the tablet and at the same time I want battery to be charging.

I think there are 3 options:

1) USB Accessory Charging Adaptor
2) Wireless charging
3) Separate USB and charging connector

Do you know any low end tablet that support at least one option?
 

Offline frozenfrogz

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Re: Tablet charging in host mode
« Reply #1 on: July 30, 2019, 08:25:14 pm »
I do not really see the problem you are talking about. All you should need is an OTG adapter and a y-connector cable with two USB A plugs and one USB A socket to connect your keyboard to (or other device).
The y-connector cable usually would have one fully connected plug and one with only +5V and GND wired in parallel. This can be hooked up to a charger / powered USB port.
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Offline gaminnTopic starter

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Re: Tablet charging in host mode
« Reply #2 on: July 30, 2019, 08:40:31 pm »
This won't work. In host mode (USB keyboard connected to the tablet) the tablet supplies 5V to the bus. There is no way to charge the tablet via the same USB port. Unless the tablet support Accessory Charging Adaptor specification where the tablet doesn't supply 5V on the bus and charges from external charger that is connected on the same bus.
« Last Edit: July 30, 2019, 08:45:22 pm by gaminn »
 

Online SiliconWizard

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Offline gaminnTopic starter

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Re: Tablet charging in host mode
« Reply #4 on: August 01, 2019, 01:48:24 pm »
Yes. Obviously, there are some mainly manufacturer specific modifications that will enable charging in host mode.

There is probably no other way than to buy/borrow some cheap tablets and try if there is a possibility to charge in host mode. And when the manufacturer discontinue that tablet you have to search again for new one with this feature.
 

Online SiliconWizard

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Re: Tablet charging in host mode
« Reply #5 on: August 01, 2019, 02:00:53 pm »
Don't think there's anything more you can do.

You can also hope that someday implementing at least 2 USB ports on tablets will become "standard", but I wouldn't hold my breath...
 

Offline joeqsmith

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Re: Tablet charging in host mode
« Reply #6 on: August 01, 2019, 02:08:18 pm »
I used a Lenovo 8" Windows based tablet to run LabView on.  There was a trick where I could short the data pins when powering up the unit.  This would put the unit in charge mode, then remove the short after it booted.  This allowed the USB to communicate.   I ended up making a custom cable for it.   Worked great.     I tried it with some new cheap Windows tablets I bought a couple of years ago and it wouldn't work with them.   

Offline gaminnTopic starter

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Re: Tablet charging in host mode
« Reply #7 on: August 01, 2019, 02:13:33 pm »
I used a Lenovo 8" Windows based tablet to run LabView on.  There was a trick where I could short the data pins when powering up the unit.  This would put the unit in charge mode, then remove the short after it booted.  This allowed the USB to communicate.   I ended up making a custom cable for it.   Worked great.     I tried it with some new cheap Windows tablets I bought a couple of years ago and it wouldn't work with them.

This is exactly what I've done with my LG G3 phone.
 

Offline angust_uk

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Re: Tablet charging in host mode
« Reply #8 on: August 01, 2019, 05:39:19 pm »
I did exactly this with a cheap Windows 10 tablet. In my case I needed to run the cables out of the back of the unit so I soldered directly to the VBUS, GND and D+/D- signals of the Micro USB socket on the PCB, injecting 5V on VBUS. This may not be the best way of doing it but it worked for me!
« Last Edit: August 01, 2019, 05:42:50 pm by angust_uk »
 

Offline Lord of nothing

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Re: Tablet charging in host mode
« Reply #9 on: August 03, 2019, 11:14:04 am »
If you not need the "Tablet" Features you can use a smal pc for who have a lot of power and can use by a plug.
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Offline NiHaoMike

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Re: Tablet charging in host mode
« Reply #10 on: August 03, 2019, 03:19:11 pm »
If you don't mind some modding, connect a 4-4.1V supply in parallel with the battery, current limited to a 1C rate or less.
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