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PS serial port line break - non functional

Former Member

I am connecting a serial port peripheral to a Zedboard, Zynq 7020, that requires a serial port line break transmission to reset the peripheral.  The Zedboard properly communicates to the peripheral through standard Linux commands cat/echo and C routines read(<fd>,..)/write(<fd>,u2026), on PS serial port 1.  However Linux, C routine, line break functions: tcsendbreak(<fd>, 0), ioctl(<fd>, TIOCSBRK, 0), ioctl(<fd>, TIOCCBRK, 0) do not reset the peripheral, nor elicit any signal on the serial transmit line.  The exact same Linux code compiled for a PC motherboard works.  Digging deeper, directly writing 0x94 to Control_reg0 (0xE0001000), clearing the STPBRK (bit 8) and setting STTBRK (bit7) should initiate the transmitter line break, however nothing happens.

Has anyone utilized the Zynq PS serial port line break feature to know if it works, or what may be implemented incorrectly?



  • 0

    Hello Paul,

     

    On the ZedBoard PS serial port 1 is connected to the Cypress USB-UART, so the break condition would need to be recognized and pass both the USB UART device and the driver on the host side. I don't know if that works or not. Perhaps someone else out there has some experience with this.

     

    Another option might be to enable the PS UART 0, connect the signals via EMIO to the PL and connect via one of the PMOD interfaces using an RS232 converter PMOD. Then you would be dealing directly with the UART without the USB-UART device and drivers.

     

    If you do find out the issue, or try the PS UART 0 let us know the results.

     

    -Gary

  • 0 in reply to Former Member

    Gary,

    Thanks for the response, I should have clarified the HW configuration, and you prompted me to verify the line break does work by bit accessing STPBRK and STTBRK bits directly.

    ttyPS1 is connected to UART 0, the device tree I acquired makes this confusing.  UART 0 is wired to EMIO and directly to PMOD pins where it is connected to PHY line drivers and the peripheral device. 

    By writing to the correct UART 0 register 0xE0000000 the serial break function does work by writing 0x94 and 0x114.  Progress!

    However do not understand why the Linux commands, tcsendbreak etc... do not work.  Will now dig into the device driver code now that HW is confirmed working.

    Paul

  • 0 in reply to Former Member

    Found the issue with the xilinx_uartps.c file preventing serial line breaks from functioning.

    The original Xilinx_uartps.c code only set STARTBRK for a line break system call.  However, STOPBRK (STPBRK) must be cleared to set STARTBRK.  This must be performed in one single write transaction.  
    Note Zynq UG585, Register Control_reg0 Details,  states STARTBRK can only be set if STOPBRK is not high, however to implement this the bits must be modified simultaneously.
    Solution:
    Update: /drivers/tty/serial/xilinx_uartps.c   Line 613:
    From: ttxuartps_writel(XUARTPS_CR_STARTBRK | status,
    To:ttxuartps_writel(XUARTPS_CR_STARTBRK | (~XUARTPS_CR_STOPBRK & status),