I have a small travel router that I no longer use, and I wanted to turn it into a NTP server.
The hardware is pretty well documented here: https://openwrt.org/toh/tp-link/tl-mr3020
For the PPS signal input into the router, I need a GPIO. I found two potential ones from the OpenWRT wiki.
GPIO7, pull down resistor R15 - 7.25kohm - datasheet pin label "GPIO7/JTAG_TDO/I2S_WS" (JTAG data output, I2S word select)
GPIO29, pull down resistor R17 - 7.25kohm - datasheet pin label "JTAG_TCK" (JTAG clock)
Before (see R15 and R17 in the middle top)
After soldering (pencil for size reference):
I put some hot glue on the wires to discourage them from moving around. I connected GPIO29 to a GPS module's PPS and connected the GPS module to the USB port for power and serial. It's all just hanging out till I cut some holes in the case.
Software details in part 2!
Ref, PPS driver document: https://code.google.com/archive/p/openwrt-stratum1/wikis/PpsGpioPollDriver.wiki
Ref, processor datasheet for the AR9331: https://www.openhacks.com/uploadsproductos/ar9331_datasheet.pdf