rtt_start() fails to auto-detect RTT control block without explicit search ranges

[fxd0h]

Environment

• Operating System: macOS 24.6.0 (Darwin)
• J-Link Model: SEGGER J-Link Pro V4
• J-Link Firmware: V4 compiled Sep 22 2022 15:00:37
• Python Version: 3.x
• pylink-square Version: Latest master branch
• Target Device: Seeed Studio nRF54L15 Sense (Nordic nRF54L15 microcontroller)
• Device RAM: Start: 0x20000000, Size: 0x00040000 (256 KB)
• RTT Control Block Address: 0x200044E0 (verified with SEGGER RTT Viewer)

Expected Behavior

The rtt_start() method should successfully auto-detect the RTT control block on the nRF54L15 device, similar to how SEGGER's RTT Viewer successfully detects and connects to RTT.

Expected flow:

1. Call jlink.rtt_start() without parameters
2. Method should automatically detect RTT control block
3. rtt_get_num_up_buffers() should return a value greater than 0
4. RTT data can be read from buffers

Actual Behavior

The rtt_start() method fails to auto-detect the RTT control block, raising a JLinkRTTException:

pylink.errors.JLinkRTTException: The RTT Control Block has not yet been found (wait?)

This occurs even though:

• The device firmware has RTT enabled and working (verified with RTT Viewer)
• The RTT control block exists at address 0x200044E0
• SEGGER RTT Viewer successfully connects and reads RTT data
• The device is running and connected via J-Link

Steps to Reproduce

1. Connect J-Link to nRF54L15 device
2. Flash firmware with RTT enabled
3. Verify RTT works with SEGGER RTT Viewer (optional but recommended)
4. Run the following Python code:

import pylink

jlink = pylink.JLink()
jlink.open()
jlink.connect('NRF54L15_M33', verbose=False)

# This fails with JLinkRTTException
jlink.rtt_start()

# Never reaches here
num_up = jlink.rtt_get_num_up_buffers()
print(f"Found {num_up} up buffers")

5. The exception is raised during rtt_start() call

Workaround

Manually set RTT search ranges before calling rtt_start():

jlink.exec_command("SetRTTSearchRanges 20000000 2003FFFF")
jlink.rtt_start()

This workaround works, but requires manual configuration and device-specific knowledge.

Root Cause Analysis

The issue appears to be that rtt_start() does not configure RTT search ranges before attempting to start RTT. Some devices, particularly newer ARM Cortex-M devices like the nRF54L15, require explicit search ranges to be set via the SetRTTSearchRanges J-Link command.

The J-Link API provides device RAM information via JLINK_DEVICE_GetInfo(), which returns RAMAddr and RAMSize. This information could be used to automatically generate appropriate search ranges, but the current implementation does not do this.

Additional Information

• RTT Viewer Configuration: RTT Viewer uses search range 0x20000000 - 0x2003FFFF for this device
• Related Issues: This may also affect other devices that require explicit search range configuration
• Impact: Prevents automated RTT logging in CI/CD pipelines or automated test environments where RTT Viewer's GUI is not available

Proposed Solution

Enhance rtt_start() to:

1. Automatically generate search ranges from device RAM info when available
2. Allow optional search_ranges parameter for custom ranges
3. Add polling mechanism to wait for RTT control block initialization
4. Ensure device is running before starting RTT

This would make the method work out-of-the-box for devices like nRF54L15 while maintaining backward compatibility.