The on-board computer consists of a microcontroller and various peripherals that allow controlling the satellite with firmware. It is essentially the brain of the whole satellite. Important parts here include the microSD that stores all the data, as well as the payload connector that allowed for communication with the cameras during the selfie-sat mission.
The OBC uses the RM46 by Texas Instruments as the microcontroller for the satellite.
The latest revision of the OBC incorporates a dual buck converter to step the 5V supply down to 3V3 and 1V2 rails to power the various subcircuits on the board.
R29, R30 and R32, R33 form resistor divider networks to set the output voltages.
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Buck converter output voltage is calculated by VOUT = VREF(1 + R29/R30) and VOUT = VREF(1 + R32/R33) where VREF = 0.8V for this particular buck converter.
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The power selection jumper is used in testing to select between the 5V input from the USB or from an external source (PC-104 pins) using a jumper.
The reverse polarity protection circuit protects the OBC from applying a reverse polarity voltage (which could be caused by a solder bridge, accidental short, user error, etc) - see here for more information.
The external oscillator for the RM46:
The 10-pin jumper here provides an option to disable the watchdog timer and pull down the CAN lines.
This is the manual reset button for the RM46 that pulls the NPORRST pin low when pressed - otherwise the NPORRST pin is tied high by R4, enabling the RM46.
The JTAG header is used for programming the RM46.