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tive componies. Application Note 38 June 2001 Universal Asynchronous
Transmitter/Receiver (UART)
Virtual Peripheral 1.0    Introduction The UART Virtual Peripheral™ uses the SX communica-
tions controller to provide asynchronous serial communi-
cation    through    an    RS-232    interface.    The    Virtual
Peripheral    provides    direct    communication    with    any
device that has an RS-232 interface, such as a PC. The
Virtual  Peripheral  has  been  developed  using  the  SX
Evaluation Board and has been tested using the SX-Key
interface  from  Parallax  Inc.  and  the  SXIDE  integrated
development environment from Advanced Transdata Inc.
Unlike other MCUs that add functions in the form of addi-
tional silicon, the SX Series uses its fast execution rate to
emulate peripheral functions in software modules, called
Virtual  Peripherals.  On-chip  hardware  peripherals  are
only provided for functions that cannot be performed effi-
ciently in software, such as timers and analog compara-
tors. 2.0    Program Description    The UART Virtual Peripheral is designed to operate at at
   a fixed frequency driven by periodic interrupts.  In the SX,
   the mechanism used to generate periodic interrupts of a
   fixed  frequency  is  the  RTCC  timer.    The  UART  virtual
   peripheral can be run as one of several threads in a mul-
   tithreaded interrupt service routine (ISR) or as part of a
   single  thread.    The  code  this  document  describes  con-
   tains a multi-tasking ISR. Whenever an RTCC interrupt
   occurs, an interrupt service routine (ISR) is called which
   contains   a   multitasker   for   allocating   CPU   bandwidth
   among  any  Virtual  Peripherals  which  require  interrupt
   service. Each task is called a thread . The UART Virtual
   Peripheral  is  split  into  a  transmit  section  and  a  receive
   section,   and   each   of   these   sections   is   assigned   to
isrThread1  . In this example, the multitasker implements 16  slots  for  calling  threads,  and  four  of  these  slots  are
occupied by calls to                                  isrThread1  . Because the UART Vir-
tual  Peripheral  only  receives  service  on  every  fourth
interrupt, most of the CPU bandwidth is available for use
by other Virtual Peripherals. 2.1 Interrupt Service Routine Flowchart Figure2-1. Interrupt Service Routine Flowchart     START Decrement Txdivide
value and check if it is 0 Transmit 1 Data bit Decrement TX count Check for any bit
to be received Receive bit     END NO YES NO YES