Unable to connect to LPC802 via SWD using ST-Link and OpenOCD

[sarveshp17]

Hello,

I am trying to program an LPC802M001JDH20 mounted on a TSSOP20 breakout board using ST-Link V2 and OpenOCD (0.12.0) on Linux, but I cannot connect to the target.

Hardware Setup

• MCU: LPC802M001JDH20

• Debugger: ST-Link V2

• Supply: 3.3 V from stlink

• Connections:

LPC802 Pin Connection

Pin 15 (VDD)	3.3 V
Pin 16 (VSS)	GND
Pin 18 (VREFP)	3.3 V
Pin 7 (SWCLK)	ST-Link SWCLK
Pin 8 (SWDIO)	ST-Link SWDIO
Pin 5 (RESET)	10k pull-up to 3.3 V

Target voltage detected by ST-Link: ~3.26 V

OpenOCD Command

openocd -f interface/stlink.cfg -f target/lpc8xx.cfg

Initial Output

Previously I received:

Warn : UNEXPECTED idcode: 0x0bc11477
Error: expected 1 of 1: 0x0bb11477

After setting the TAP ID:

openocd -f interface/stlink.cfg -c "set CPUTAPID 0x0bc11477" -f target/lpc8xx.cfg

I now get:

Error: init mode failed (unable to connect to the target)

Observations

• When OpenOCD runs, an LED connected to a GPIO (pin 4 on tssop-20) briefly lights and then turns off.

• RESET pin measures about ~1.3 V instead of 3.3 V.

• Voltage between VDD (pin 15) and RESET (pin 5) is about 2 V.

• The MCU has never been programmed before (new chip).

Questions

1. Should the RESET pin normally sit at VDD internally, or is an external pull-up mandatory?

2. Could the abnormal RESET voltage indicate the chip is stuck in reset?

3. Is there anything else required for minimal LPC802 SWD debugging (boot pins, ISP mode, etc.)?

Any help would be appreciated. Thank you.

[Harry_Zhang]

Hi @sarveshp17 

The LPC802 cannot be reached by SWD because the RESET pin is not pulled high. The RESET pin has no internal pull‑up, so it should have an external 10 kΩ pull‑up to 3.3 V. Your measured RESET voltage (~1.3 V) means the MCU is stuck in reset, so SWD cannot connect.

You can refer to the lpcxpresso804 schematic.

BR

Harry

[sarveshp17]

Thank you @Harry_Zhang for your super fast guidance!

After adding the 10 kΩ pull-up to the RESET pin, the chip is now successfully detected by OpenOCD.

However, I’m still facing another issue. When I try to connect GDB through OpenOCD, or when I attempt to retrieve flash information via telnet, OpenOCD reports an “unknown Part ID” and then stops responding. I’ve also noticed that the Part ID value is inconsistent across attempts.

I’ve attached the relevant screenshots for reference.

Is there something else I might be missing?

Thank you !!!

[Harry_Zhang]

Hi @sarveshp17 

the remaining issue seems to be unstable SWD signals or the RESET line being pulled low unintentionally. I think you can  re-check the RESET wiring, shorten SWD cables, lower the SWD clock , and enable “connect under reset”.

Can  you share the SWD schematic？

BR

Harry