Scenix full duplex software UART Virtual Peripheral

The program "fduart.asm" contains a high-speed full duplex software UART virtual peripheral for the Scenix SX microcontroller. It also demonstrates techniques for implementing circular buffers, hexadecimal output, text message output, and multiplexed use of I/O pins for both switches and LEDs.

The program runs on the Parallax SX-Key demo board, and must be assembled with the Microchip MPASM assembler, using the "/c-" command line option. The SX runs at 50 MHz with turbo mode enabled, and handles a serial rate of 115.2 Kbps.

Updated 4-Nov-1998: fixed bug in handling of transmit buffer full flag.

The program uses a 3x sampling technique described on my Notes on bit-banging async serial page.

Permission is granted to use this code or portions thereof for non-commercial purposes provided the copyright notice is preserved intact and credit is given to the author. For any other use, a license may be obtained from the author; send email to

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Last updated August 4, 2003

Copyright 1998, 2003 Eric Smith

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