SWD-Port of the Cortex-M0: Cannot connect to MEM-AP on Cortex-M0 Designstart Eval

The “SWD-Port of the Cortex-M0: Cannot connect to MEM-AP on Cortex-M0 Designstart Eval” error indicates an issue with connecting to the memory access port (MEM-AP) when trying to debug or program a Cortex-M0 microcontroller over the Serial Wire Debug (SWD) interface. This is commonly seen when using development boards like the DesignStart Eval board with ARM’s Cortex-M0 processor.

The root cause is usually a configuration issue preventing the debug probe (like J-Link or ST-Link) from accessing the MEM-AP registers and memory on the target microcontroller. The MEM-AP provides the interface for reading, writing, and modifying memory when debugging firmware. Without access, it is not possible to flash new programs or step through code.

Potential Causes

There are a few common reasons why the SWD interface may fail to connect to the MEM-AP on a Cortex-M0 microcontroller:

• Incorrect target voltage – The debug probe and target board must be at the same logic-level voltages for SWD communication. Double check that the probe and board operate at either 1.8V or 3.3V as required.
• Faulty USB connection – Try disconnecting and reconnecting the USB cable between the debug probe and host computer. Check for loose connections or damaged cables.
• Outdated probe firmware – Update to the latest firmware on the J-Link, ST-Link, or other debug probe. Old firmware versions may be incompatible with the Cortex-M0 target.
• Incorrect debug interface – On some boards, the SWD pins are mislabeled or routed improperly. Verify using the board schematic that the correct pins are wired to the debug probe.
• Target not in debug mode – The microcontroller reset pin should be asserted and debug mode enabled for SWD MEM-AP access. Verify the target reset circuitry and debug pin configuration.
• Faulty target chip – A defective Cortex-M0 chip itself can also prevent SWD communication in rare cases. Try substituting a known good microcontroller if possible.

Steps to Resolve

If encountering the “cannot connect to MEM-AP” error, work through these debugging steps:

1. Check voltage levels – Use a multimeter to verify the target board and debug probe are at the expected voltage. Either 1.8V or 3.3V operation must match.
2. Try different USB ports – Eliminate any USB issues by trying different host ports and cables between the debug probe and computer.
3. Update debug firmware – Get the latest firmware from the probe vendor’s website and update via the utility software.
4. Inspect schematics – Consult the development board schematics and confirm the SWD pins are correctly connected to the debug probe header.
5. Verify reset state – Check that the target’s reset line is asserted and debug mode is set during SWD access.
6. Swap target chip – As a last resort, replace the Cortex-M0 microcontroller with a verified working unit if possible.

Bypassing MEM-AP for Programming

If the SWD interface is able to establish a basic connection but cannot access the MEM-AP, there are some workaround options to program the target Cortex-M0 flash memory without using the MEM-AP:

• JTAG Interface – Some debug probes also support JTAG debugging which does not use the MEM-AP. Switch over to JTAG mode if available.
• Direct Flash Programming – Certain tools allow directly flashing the internal flash memory over SWD without accessing the MEM-AP. Consult your debugger’s documentation.
• External Flash Chip – Configure the microcontroller to boot from an external SPI flash chip. Program the external flash over SPI since SWD is not available.

While cumbersome, these techniques can sometimes succeed in programming a Cortex-M0 target without fixing the root MEM-AP access issue over SWD. But it is recommended to debug and correct the MEM-AP connection problem for normal SWD-based development.

Preventing MEM-AP Connection Issues

For robust and repeatable Cortex-M0 development, follow these design and configuration practices when using SWD debugging:

• Carefully follow chip vendor guidelines for target board design, especially SWD signal integrity and reset control.
• Use quality USB cables between debug probe and host computer to eliminate connectivity problems.
• Verify target and probe voltage levels match at either 1.8V or 3.3V as required.
• Check board schematics thoroughly and confirm correct SWD pin mapping to debug headers.
• Keep debug probe firmware updated by installing new releases from the vendor.
• Use proper decoupling capacitors for power integrity to the target microcontroller.
• Follow recommended layout practices in the Cortex-M0 user guide for the SWD interface.

Paying close attention to debug interface design, following vendor guidelines, and keeping tools up-to-date will help avoid issues connecting to the MEM-AP over SWD during Cortex-M0 development.

Conclusion

The “SWD-Port of the Cortex-M0: Cannot connect to MEM-AP on Cortex-M0 Designstart Eval” error indicates the debug probe cannot access the Cortex-M0 memory access port registers and memory over the Serial Wire Debug interface. This prevents flashing new firmware or debugging code on the target microcontroller. Typical causes include voltage mismatches, faulty cables, incorrect pin mapping, or outdated probe firmware. By methodically checking voltages, cables, schematics, and firmware updates, the root cause can usually be found. Workarounds like direct flash programming over SWD or using JTAG may succeed without fixing the MEM-AP issue, but it is recommended to debug and solve the MEM-AP connection problem for smooth Cortex-M0 SWD-based development.