LPC4088 QuickStart Board - How to Expand - UART


There are four UART interfaces on the LPC4088 QuickStart Board. The UART interface is perfect for communication between separate controllers over longer distance but also local communication with RF modules. UART channels are typically point-to-point connections but there are ways to create buses like the RS422/485 interfaces, see example schematics.

  1. UART0 is available on p41/p42, not note that this is the HDK communciation channel also. The HDK connection can be overridden, if needed.
  2. UART2 is available on the XBee comatible interface.
  3. UART3 is available on p9/p10.
  4. UART4 is available on p31/p37.

The advantages of a UART channel:

  • Asynchronous communication, no clock signal needed.
  • Full duplex communication (simultaneous communication in both directions).
  • Only two signals needed.
  • With transceivers and differential signaling, reliable communication over long distance is possible.

The disadvantages are:

  • Simple protocol. No error handling. All intelligence must be implemented in software.
  • Asynchronous communication limits the maximum reliable bit rate to about 200 kbps.

For more information about UART, see http://en.wikipedia.org/wiki/Uart
For more information about RS232, see http://en.wikipedia.org/wiki/RS-232
For more information about RS422, see http://en.wikipedia.org/wiki/RS-422
For more information about RS485, see http://en.wikipedia.org/wiki/RS-485
For more information about MIDI, see http://en.wikipedia.org/wiki/MIDI
For more information about DMX512, see http://en.wikipedia.org/wiki/DMX512-A#DMX512-A

The UART channels can be used simultaneously. Although not commonly used in reality it is possible for one UART channel to be shared by different parts of the application program, for example when running several processes/tasks in an RTOS (Real-Time Operating System). Some kind of mutual-exclusive protection or driver abstraction must then be created so that the use of the bus is serialized in time, i.e., two or more users do not try to use the bus simultaneously.
More typical is that different parts of the program use separate UART channels.

Example Schematic

Three pages covers example UART/RS232/RS422/RS485/MIDI/DMX512 expansion in the example interface design schematic.


  • The UART0 interface that is available on p41/p42 is connected to the HSK communication. If the HDK is powered down (by removing JP3) or is a series resistors is connected to the receive signal the HDK connection can be overridden, if needed.
  • In RS422/485 buses with multiple drivers a software protocol layer must control which device that is allowed to actively drive the bus.

Example Code


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