AS UNO 8
A picture is worth a 1000 words they say:
Note the lack of a crystal, USB Serial chip, and LEDs.
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No crystal - This board uses the Internal Oscillator of the ATmega328P - available in 1, 2, 4 and 8MHz options. I strongly suggest that you use MCUdude's MiniCore board package - JSON file is at https://mcudude.github.io/MiniCore/package_MCUdude_MiniCore_index.json
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No USB Serial - The female header at the end of the board is to accept a CP2102 USB-UART module. This will allow you to upload sketches just like a regular UNO or Nano, but doesn't drain you battery once it is removed. It also provides regulated +5 and 3V3.
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No LEDs - There are pads to install a 3mm Power LED and 'L' LED to the right of the MCU. For if you don't want to waste juice on pretty lights.
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Note that this board will run on lower voltages to further conserve current drain. At 8MHz it will run happily at 3.3V, at 1MHZ you can get away with 1.8V.
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5 Sel 3 - This is a jumper/solder pad to select one of 2 Input voltages from the CP2102 connector. Jumper or solder short either 5 or 3 to the 'Sel' pad. I you don't short anything - nothing will work. Conversely you could use 'Sel' as a Vin pin.
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0 .... 7 - these are Arduino pins D0 thru D7 as in digitalWrite( 4, HIGH ), they form the 'D' bus. Pins 0 and 1 correspond to Serial Tx and Rx respectively.
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Reg 3V3 - This is for a Regulator module - more on this at a future date.
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LEDs - 2 sets of pads are provided for installing LEDs to the right of the MCU. 'P' is for a Power LED and 'L' is for the Arduino LED_BUILTIN on pin D13. They have higher value resistors in series to keep from blinding you as you marvel at the board during development.
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ICSP - 2x3 Header - this is the InCircuit Syste Programmer interface used to directly program the chip via an USBasp. They are available with 10 pin to 6 pin adapters. Pin 1 - indicated with a square pad on the bottom of the board is MISO - This bus is the SPI bus. If you upload a sketch with an USBasp you will lose the bootloader - your sketch will start immediately but you will not be able to upload via the USB Serial until you re-install a bootloader with the USBasp. The clock frequency (and other fuses) of the chip are set during the bootloader burn process. Be sure to select the Internal Oscillator before burning... There is no external crystal.
