© 2001 Ubicom, Inc. All rights reserved. - 9 - www.ubicom.com Asynchronous Transmitter & Receiver (UART) Virtual Peripheral Implementation AN38 4.0    Baud Rate Generation and Timing As an example of calculating the parameters which con-
trol  the  timing  of  the  UART  Virtual  Peripheral,  consider
transmitting data at 14400 baud with four times oversam-
pling (i.e. a sampling frequency of 57.6 kHz).
Transmission time for 1 bit = 1/14400 seconds
The divide ratio                           UARTTxDivide   for the above example is
the sampling rate divided by the baud rate. The sampling
rate is defined by the interrupt frequency and the number
of slots for the UART virtual peripheral ISR in the multi-
tasker.
So  the  formula  for  the UARTTxDivide    and UARTRxDi- vide constants   is: UARTTxDivide = UARTTxFs/(UARTBaud)
= 57600/(14400 * 4) = 1 In receive mode, the generation of baud rate is calculated
in the same way, except that a constant called                                                                                     UARTRx-
StDelay   is used to skip over the start bit. This constant is equal  to  1.5  times  the  baud  period.  Its  purpose  is  to
ensure that the bits are sampled near the middle of each
pulse. 4.1           Circuit Design Procedure
The simplest version of the circuit requires two port pins
for  transmit  and  receive.  If  hardware  handshaking  is
used,  additional  port  lines  are  required.  The  hardware
interface only requires a driver for converting the voltage
level  of  the  signals.  The  same  concept  can  be  used  to
extend and configure two or more independent UARTs.