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.
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).
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.
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.
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.
