MCU Link with custom LPC5514 board

[wandaWilliams]

I have a custom board with an LPC5514 and I am trying to connect the MCU Link to it over the SWD interface. My understanding is that all we need are:

• VCC (3.3V)
• GND 
• RESET (pin 32)
• SWDIO (PIO0_12)
• SWCLK (PIO0_11)

I have powered on the board (3.3V is stable),

• GND from the MCULink is connected to the GND on my board,
• IF_RST from MCU_Link is connected to RESET on my board
• IF_SWDIO from MCU_Link is connected to SWDIO on my board
• IF_SWCLK from MCU_Link is connected to SWCLK on my board

I am not seeing any response MCUXpresso other than that the target is not connected. 

When not connected, I am seeing activity on the IF_SWCLK and IF_RST pins on the MCU_Link, so I know that the MCU_Link works. But when I connect it to my board, something pulls them low, even when the IF_RST pin is high. It looks like something is not letting the LPC5514 come out of reset, but at the same time, I am not seeing any activity on SWCLK when the two boards are connected. 

What could I be missing?

[wandaWilliams]

Hi @danielholala 

The only pin that is pulled up on my board is the RESET pin with a 10k resistor.

[danielholala · Accepted solution]

Hi Wanda,

have a look at AN13033 "Hardware Design Guidelines for LPC55(S)xx Microcontrollers".

Chapter 6 provides a sample schematic for the connecting debug signals:

The pull up/down resistors are recommended. Give it a try and add them. 

[wandaWilliams]

Hi @danielholala, this is super helpful. I used 15k resistors as that is what I have ready access to. It definitely gets me to see changes on the signals. However strangely the RESET pin is toggling like a clock. I had expected (like in SPI) that the RESET pin to stay low (and SWCLK) for the duration of the SWDIO transaction. It looks like I have more work to do; but this is super helpful. Thank you!

[danielholala]

No pullup/pulldowns? Maybe you could reproduce the relevant part of the schematics for the SWD interface.