[Example Script] swo_read_stimulus() returns empty List. Even if num_bytes is not zero.

[cogito666]

import pylink

try:
    import StringIO
except ImportError:
    import io as StringIO
import string
import sys
import time


def serial_wire_viewer(jlink_serial, device):
    """Implements a Serial Wire Viewer (SWV).

    A Serial Wire Viewer (SWV) allows us implement real-time logging of output
    from a connected device over Serial Wire Output (SWO).

    Args:
      jlink_serial (str): the J-Link serial number
      device (str): the target CPU

    Returns:
      Always returns ``0``.

    Raises:
      JLinkException: on error
    """
    buf = StringIO.StringIO()
    jlink = pylink.JLink(log=buf.write, detailed_log=buf.write)
    jlink.open(serial_no=jlink_serial)

    # Use Serial Wire Debug as the target interface.  Need this in order to use
    # Serial Wire Output.
    jlink.set_tif(pylink.enums.JLinkInterfaces.SWD)
    jlink.connect(device, verbose=True)
    # jlink.connect(chip_name='NRF52832_XXAA', verbose=True)


    jlink.coresight_configure()
    jlink.set_reset_strategy(pylink.enums.JLinkResetStrategyCortexM3.RESETPIN)

    # Have to halt the CPU before getitng its speed.
    jlink.reset()
    jlink.halt()

    cpu_speed = jlink.cpu_speed()
    swo_speed = jlink.swo_supported_speeds(cpu_speed, 10)[0]

    # Output the selected SWO speed to the console
    sys.stdout.write(f'swo_speed: {swo_speed}\n\n')
    sys.stdout.write(f'swo_speed: {swo_speed}\n\n')

    # Start logging serial wire output.
    jlink.swo_start(swo_speed)
    jlink.swo_flush()

    # Output the information about the program.
    sys.stdout.write('Serial Wire Viewer\n')
    sys.stdout.write('Press Ctrl-C to Exit\n')
    sys.stdout.write('Reading data from port 0:\n\n')

    # Reset the core without halting so that it runs.
    jlink.reset(ms=10, halt=False)

    # Use the `try` loop to catch a keyboard interrupt in order to stop logging
    # serial wire output.
    try:
        while True:
            # Check for any bytes in the stream.
            num_bytes = jlink.swo_num_bytes()
            sys.stdout.write(f'num_bytes: {num_bytes}\n\n')

            if num_bytes == 0:
                # If no bytes exist, sleep for a bit before trying again.
                time.sleep(1)
                continue

            data = jlink.swo_read_stimulus(0, num_bytes)
            # data = jlink.swo_read(0, num_bytes)
            # sys.stdout.write(f'data: {data}\n\n')

            sys.stdout.write(''.join(map(chr, data)))
            sys.stdout.flush()
    except KeyboardInterrupt:
        pass

    sys.stdout.write('\n')

    # Stop logging serial wire output.
    jlink.swo_stop()

    return 0


if __name__ == '__main__':
    exit(serial_wire_viewer(sys.argv[1], sys.argv[2]))

I am using upper script for nRF52DK.

When I run the script, num_bytes is not zero.
But the swo_read_stimulus() returns empty list.
What's wrong? Am I missing something?

I am using nrf52832 target CPU and it's programmed using Zephyr RTOS.

[hkpeprah]

I don't think I have used swo_read_stimulus(); the example application is for use with swo_read().

[cogito666]

I don't think I have used swo_read_stimulus(); the example application is for use with swo_read().

Oh, thanks for letting me know.
The previous code from document.. Maybe it's out dated.
https://pylink.readthedocs.io/en/latest/swd.html

Anyway as you mentioned, I tested with the script in Example.
Below is the log in my IDE (PyCharm)

---

Serial Wire Viewer
Press Ctrl-C to Exit
Reading data from port 0:

ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running
ERROR:pylink.jlink:Cannot read register 0 (R0) while CPU is running

Is this log indicates that my CPU is locked? or has it something to do with the APP-PROTECT?

`

[hkpeprah]

The SWD example is for cases where you are using SWD in your target to write data when a debugger is attached. Your firmware must be using SWD debug, and implementing write() to trigger a vector catch. There's an example here in the semihosting section on Memfault's blog of doing that: https://interrupt.memfault.com/blog/printf-on-embedded#semihosting

// Based on the implementation here:
// https://github.com/bminor/newlib/blob/1a0908203606527b6ac0ed438669b5bcd247a5f9/newlib/libc/sys/arm/syscalls.c#L314-L338
// And here:
// https://github.com/bminor/newlib/blob/69f4c4029183fb26d2fcae00790881620c1978a3/newlib/libc/sys/arm/swi.h#L74-L89
int _write(int file, uint8_t *ptr, int len) {
  int data[3] = {file, (int) ptr, len};
  __asm__("mov r0, %0 \n mov r1, %1 \n bkpt #0xAB"
          : : "r" (5), "r" (&data) : "r0", "r1", "memory");
  return len;
}

The SWV is used for Serial Wire Output (SWO), and similarly, you must implement a _write() in your firmware that outputs data to SWO. You also need to assign a GPIO for use with SWO. They have an example section on that as well: https://interrupt.memfault.com/blog/printf-on-embedded#serial-wire-output-swo