What are these exposed copper rectangles for on the mbed NXP LPC1768?

Question

Today I noticed some exposed copper rectangles on the bottom of an mbed NXP LPC1768 dev board. They don't look like they're meant for components. I think they may just be test points, but I'm curious if there's another answer.

Here's an image of the board with the copper rectangles circled in red:

I'm not sure what the mbed interface chip is. Googling it suggests that it's proprietary. I can't tell where the traces are going.

Possible duplicate of [What use do these round pads have on PCB?](https://electronics.stackexchange.com/questions/173068/what-use-do-these-round-pads-have-on-pcb) — Dmitry Grigoryev, Oct 30 '18 at 07:55

score 10 · Answer 1 · answered Oct 29 '18 at 18:52

10

NXP has the schematic for this board, it looks like those pads make up a programming interface for that chip, which as you say is likely custom in some way.

answered Oct 29 '18 at 18:52

Phil G

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score 8 · Answer 2 · answered Oct 29 '18 at 18:50

Those look like test pads and seem large enough for soldering a wired connection.

An educated guess would be JTAG (TCK, TDI, TDO, TMS) plus VCC and GND. VCC and GND could be verified with a multimeter.

Note that the interface chip is also a programmable microcontroller, and thus needs JTAG or SWD for production programming.

score 3 · Answer 3 · answered Oct 29 '18 at 18:49

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Sometimes, you see such elements on microwave boards on the microwave lines – then, they serve the purpose of being a capacitive or reactive component (or both); but since this board definitely doesn't look like an upper-GHz RF board:

These are almost certainly test points.

answered Oct 29 '18 at 18:49

Marcus Müller

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Microwave elements require controlled impedance traces, which you don't see on this board. – Turbo J Oct 29 '18 at 18:52
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pretty much my words :) – Marcus Müller Oct 29 '18 at 18:54

score 3 · Accepted Answer · answered Oct 30 '18 at 07:06

Adding to the answer of phill g: What are these exposed copper rectangles for on the mbed NXP LPC1768? which provides the schematic of that board at https://www.nxp.com/downloads/en/design-support/ARM_mbed_LPC1768_Schematic.pdf

In the schematic they're even designated as the cfg0-cfg5 pads of MBED-IF01 chip. On https://os.mbed.com/questions/76861/mbed-IF01/ on a question regarding the datasheet of that part, it is stated that:

IF01 is the the interface circuit of the LPC1768, which infact is an "LPC2148" MCU, in short we can't open source a lot of the information for the interface so this is why its hidden.

It seems to be the predecessor of https://os.mbed.com/handbook/mbed-HDK

It's a microcontroller implementing https://os.mbed.com/handbook/cmsis-dap-interface-firmware

The CMSIS-DAP Interface Firmware provides:

USB Mass Storage Device for drag and drop programming of the target chip

USB Communications Device Class for Serial Communication with the target chip

USB HID CMSIS-DAP for debugging

USB bootloader for updating the interface firmware itself

As to what those pads really are, when you look in the datasheet of the LPC2148 https://www.nxp.com/docs/en/data-sheet/LPC2141_42_44_46_48.pdf we can see that those pins are in fact the tdo, tdi, trs, trst and rtck pins.

Those pins are used thus to flash their custom cmsis-dap interface to that chip (probably using pogo pins).

What are these exposed copper rectangles for on the mbed NXP LPC1768?

4 Answers4