A complete Arduino library for the MAX3100 serial UART/USART chip can be downloaded below. There have been many requests for an Arduino library for the MAX3100 external serial USART (UART) chip from Maxim Integrated. As an example of the minor changes needed to use the MAX3100's serial port instead of (or in addition to) the built in serial port, here is some sample code (note the lower case 's' is the only change needed to use the new serial port).
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A one stop shop for using the ATMEGA328P standalone at 8MHz or 1MHz clock speed including compiled bootloader. This is for use primarily on a breadboard, but also also directly on, say, a Uno R3 or Nano.