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There is this saying that the USB-Port of the Pi(or the pi itself somewhere in it) had issues with synchronous data transmission, like sending and receiving data via bluetooth dongle at the same time.

Is that true and what is really to it? Isn't that just a power issue and if you give it a proper power supply, it won't run into it?

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Introduction

USB on RaspberryPi is quite a complicated topic. USB in general is very complicated bus, with many modes of operations and possible configurations. It's not often but from time to time it happens that some USB device won't work with some USB host while it is working without problems with other USB host (like other PC computer). Problem is - with RasbperryPi, comparing to other USB host, it's much easier to find a device that has problems.

Some more details

So there are two separate issues here:

Power Problems

RaspberryPi does not deliver enough power for some devices. Using powered (active) Hub solves most of this problems.

Protocol problems

Usb host device in RaspberryPi is not the best one and it has a lot of problems. Good host devices provide a lot of hardware support for many USB bus features while the chip used in RasbperryPi does not. Unfortunately it seems that it just wasn't designed to be used as reagular USB host connecting many complicated USB devices.

There is a lot of ongoing effort to fix all the problems in software, though. There is one RaspberryPi Foundation's employee working full time just on that right now and there are some other volunteers helping. It seems that the progress is quite good and we already have some fixes in the official firmware. More patches are available, however, but they are much more intrusive and are not yet considered stable enough to be pushed to the official firmware yet. This should change in very near future, though.

Conclusion

I consider USB host to be the weakest part of the RasbperryPi device. The problems where especially visible at the beginnings of RasbperryPi and are being constantly improved over the time. This means that many informations about USB problems on RaspberryPi you can find on the Internet may be outdated right now or will be outdated in very near future. That being said, it is unknown if all the problems will ever be fixed.

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Cannot vouch for your particular BT adapter, but my RasPi has an external HDD connected directly to the USB port without any extra power supply, that regularly achieves 5-8mb/s transfer speeds over a multi-gigabyte transfers, that are read over the network (Ethernet is also basically connected to the same USB due to RasPi architecture). Haven't seen any issues yet.

  • It is about synchronous transmission. Also it is unclear how much current your hdd needs. So basically your setup is no proof for or against this theory. – Zurechtweiser Jun 7 '13 at 6:52
  • @Zurechtweiser HDD is rated .5A max, and takes probably 1/2 or 1/3 of that during the normal operations, and pretty much saturates USB bandwidth in both directions. you may easily have a very picky BT dongle, that requires some particular USB configuration. – lenik Jun 7 '13 at 7:30
  • What kind of voltage supply do you have(Amps)? "pretty much saturates USB bandwidth in both directions" How do you know? – Zurechtweiser Jun 7 '13 at 7:33
  • @Zurechtweiser I use standard power supply, that comes with RasPi, something like 5V 2A. Regarding the USB being the bottleneck, the HDD can read/write about 20-30mb/sec, the 100Mbps Ethernet supports up to 12mb/sec and I don't see any other limiting factor except USB. What's your BT expected bandwidth, over 8mb/sec? – lenik Jun 7 '13 at 8:17

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