[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.