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USB Low Speed bandwidth?

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USB Low Speed bandwidth?

星期二
82 次查看
Hoditohod
Contributor I

I have an embedded system where 2 microcontrollers communicate over Full Speed USB bus. The host is an iMX RT1060, the device is an LPC55S69. From EMI standpoint it would be beneficial to use Low Speed instead of Full Speed. The traffic on this link is 8-10kbyte/sec, and I thought this is fine with USB LS 1.5Mbit/sec signaling rate.

But as found out USB LS has other restrictions compared to FS:

  • only Interrupt and Control transfer types are available
  • Interrupt poll rate is 10ms
  • packet size is 8 bytes
  • ACM class not available, only HID

With this the effective bandwidth drops to <1kbyte/sec (100Hz * 8bytes). This seems really slow compared to the signaling rate, it's slow even compared to a plain UART.

Is there a way to push more data through a USB LS link?

I have control over the USB drivers on both side, the devices are hard-wired together (no need to be compatible with anything else), going slightly off the USB specification is ok.

Would it be possible to increase the poll rate, packet size or implement a custom class to get around 10kbyte/sec bandwidth?

I'm not an expert on USB, but I'm willing to dig into the topic. The question is, should I invest time time in it, or is it a lost cause?

How would you do it?

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