© 2001 Ubicom, Inc. All rights reserved. - 4 - www.ubicom.com AN38 Asynchronous Transmitter & Receiver (UART) Virtual Peripheral Implementation 3.3 Initialization Section This  section  provides  initialization  for  the  UART  Virtual
Peripheral. This has to be included with the initialization
of all other ports and registers, prior to entering the main
loop of the application. _bank rs232TxBank ;select rs232 bank mov w,#UARTTxDivide    ;load  Txdivide ;with UART baud
;rate mov rs232TxDivide,w Initialization  is  required  to  send  the  data  at  the  desired
baud rate. The value of UARTTxDivide   specifies the num- ber of times the thread has to be serviced before a bit is
transmitted.  For  example,  at  9600  baud  the  value  of UARTTxDivide   is 6 (57600/9600=6), which means that a bit is transmitted once for every six times isrThread1    is
called. 3.4 Interrupt Section This section is the UART Virtual Peripheral ISR. The flow
of  the  interrupt  service  routine  is  demonstrated  by  the
flowchart in Figure2-1.
The ISR returns with a "retiw" value of -217 every 4.32
microseconds at an oscillator frequency of 50 MHz. The events that occur on an interrupt are:
1.An interrupt occurs whenever the RTCC value rolls      from $FF to $00. The interrupt jumps to the ISR at $00.
2.In the ISR, the value of isrMultiplex    is incremented. Initially it is zero. isrMultiplex   implements rotation among the slots for threads called by the multitasker. isrMultiplex   is added to the value of the program      counter to jump into a table of jump instructions for the
     threads.
3.In isrThread1   , the UART Virtual Peripheral ISR is ex-      ecuted.
4.The value of rs232TxDivide    is checked for zero, to confirm whether a bit has to be transmitted in this cycle.
The value of                        UARTTxDivide   is loaded to
rs232TxDivide   . 5.The value of rs232TxCount    is checked to confirm the      presence of data to be transmitted. If the value is not
     zero, there is data to be transmitted, in which case the
     transmit routine is executed.
6.The data stored in rs232TxHigh    is pushed to the W      register.
7.The MSB of the rs232TxLow    register is set, which is the      start bit. A total of ten bits are transmitted, which con-
     sists of 1 start bit + 8 data bits + 1 stop bit. The receiver
     has to be configured for this format.
8.The bits are rotated to the right and fed to the rs232TxLow   register. 9.Bit 6 of the rs232TxLow    register is transmitted on the      TX pin.
A  similar  procedure  is  used  to  receive  the  incoming
bytes.