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Serial port drops bytes when USB HID device is connected

A. Kirchhoff
Intellectual 535 points
Other Parts Discussed in Thread: OMAP-L138

I have an OMAP-L138 and an MSP430 communicating using their UARTs at 115,200 baud.  The MSP430 is sending a 27-byte data packet every 13.5 milliseconds.  While the MSP430 is sending data packets, I connect a USB HID device (I have tried a GPS and a mouse) and errors occur when parsing a couple of the data packets.  An extra byte equal to 0 appears after the first byte of the packet and only the first 16 bytes of the packet are received.  I have confirmed that the bytes are missing when they enter my application via a system read of /dev/ttyS1 (i.e. the errors are not a bug in my application).  The same errors occur when I disconnect the USB device.

Any thoughts on what is going on?  The UART has a 16-byte FIFO in hardware, which is enabled.  Is a USB interrupt taking too long?

Any ideas on solutions?  The hardware can't change, so I can't add HW flow control.

The OMAP-L138 is clocked at 300 MHz and is running the 2.6.33 Linux kernel (DaVinci PSP 02.20.00.12).

Thanks,

abkirchhoff

  • 0
    Intellectual 535 points

    Can anyone confirm that this is an issue?  Is the USB interrupt the problem or just the system load when a USB device is plugged in?  I would like to understand the problem more fully before I work on a solution.

    I temporarily reduced my packet size from 27 bytes to 12 bytes and no errors occurred when using the 12-byte packet.  I assume this is because the packet fits within the 12-byte FIFO in the UART hardware module.

    Any help would be appreciated.

    Thanks,

    abkirchhoff