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This is a commonly available relay board with a USB HID-like interface.Two relay board with a USB interface

I haven't been able to find a datasheet for it on the sites that sell it. On its right side there's a four pin header. I'm curious regarding its functionality. The PCB traces from the header on the bottom of the board lead to through-holes near the IC. I measured its connectivity and it does not appear to be a mirror of the USB port. Any ideas?

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    \$\begingroup\$ Maybe some debug, or firmware loading interface. That would be easier to say if you gave the chip reference and the pins to which these signals are connected to. \$\endgroup\$ – dim Mar 19 '20 at 22:28
  • \$\begingroup\$ Good idea! The markings on the chip are: Atmel 20U — TINY45 — 1913VSR (The last part I would think is the date / batch code.) I will have to power down the board to measure the pin connectivity. Will try to do so later. \$\endgroup\$ – Diomidis Spinellis Mar 19 '20 at 22:49
  • \$\begingroup\$ Many thanks! I was able to answer the question using your suggestion. \$\endgroup\$ – Diomidis Spinellis Mar 20 '20 at 9:41
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The four header pins are connected to pins 1, 7, 6, 5 of the Atmel microcontroller. These can function as RESET, SCK, MISO, MOSI for serial programming the EEPROM by pulling RESET to low.

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