Dodgy serial port suspected

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    Michael Sacarny

    I have been using a BerryGPS with IMU v4 on RaspPi for several months and began to notice that GPS output (eg – by screen /dev/serial0) would occasionally go from clear to garbled and back to clear. The garble seems to have gotten much worse of late, echo to send U-Blok commands does not seem to work, and u-Center connects very rarely and does not seem to be able to send commands. This is all at 9600 baud.

    I suspect either the BerryGPS or RaspPi UART is the problem. I have resoldered and reseated my BerryGPS connector pins and will change up the RaspPi but are there any other diagnostics?


    Michael Sacarny

    I discovered that there is a problem with the RasPi UART clock scheme that results in variable UART clocking and subsequent corruption, described here.

    “The mini UART port can be accessed using the /dev/ttyS0 device in Linux. One issue with the mini UART controller is that its baud rate is linked to the VPU core frequency so as the VPU frequency changes depending on processor load so does the baud rate of the UART controller. This will cause problems as serial devices expect a constant baud rate during communication.”

    Using the PL011 UART rather than the mini resolved my problem. I wish I had known about this before expending so much time on it.

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