© 2001 Ubicom, Inc. All rights reserved.
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AN38
Asynchronous Transmitter & Receiver (UART) Virtual Peripheral Implementation
3.0 Source Code Sections
The source code for the UART Virtual Peripheral is
divided into five sections:
•Equates Section: Constants that define the operation
of the UART
•Bank Section: Memory allocation for the virtual periph-
eral
•Initialization Section: Must run before the mainline ap-
plication code can run
•Interrupt Section: The most important part of the virtual
peripheral; the section of code that continually runs
(while enabled) and mimics hardware in software
•API Subroutines: Send data to and receive data from
the Virtual Peripherals. Analogous to the API routines
that would be written for accessing hardware, and
make the operation of the virtual peripheral transpar-
ent.
When integrated into an application, each section of the
source code is inserted at an appropriate location in the
main body of the application’s source code.
3.1
Equates Section
The equates section provides configuration constants,
allowing compile-time changes in the functionality of the
UARTs.
UARTRxFs ,
UARTTxFs : These constants define the SAM-
PLING RATE of the UART, and must be calculated by the
software programmer based on the Interrupt Frequency
and on how the
ISR-Multitasker is set-up. In this ver-
sion of the source code, the UART transmitter and
receiver are both sampled at rates of 57600Hz.
InterruptFrequency=
UARTTxFs =
By Default,
oscillator frequency = 50MHz
Interrupt frequency » 230400Hz
Number of threads calling the rs232Transmit/Receive
routines = 4
Number of ISR Threads is 16.
----------------------------------------------------------
Therefore,
UARTTxFs and
UARTRxFs = 57600.
The multitasker rotates interrupt service among 16 slots,
and isrThread1 is called from four of these slots. In
other examples, one slot of 16 might be sufficient to ser-
vice the UART Virtual Peripheral ISR.
The pins on which the input and output data are received
and sent are also defined in this section. Port A is used
for the external interface.
The port pins are configured as follows:
• ra.0
rs232RTSpin
• ra.1
rs232CTSpin
• ra.2
rs232Rxpin
• ra.3
rs232Txpin
intPeriod
= 217
UARTfs
= 230400
Num
= 4
IFDEF baud1200
UARTBaud
= 1200
ENDIF
IFDEF baud2400
UARTBaud
= 2400
ENDIF
IFDEF baud4800
UARTBaud
= 4800
ENDIF
IFDEF baud9600
UARTBaud
= 9600
ENDIF
IFDEF baud1920
UARTBaud
= 19200
ENDIF
IFDEF baud5760
UARTBaud
= 57600
ENDIF
IntPeriod
Frequency
Oscillator
threads
ISR
of
number
total
Transmit
rs
calling
threads
number
frequency
nterrupt
i
232
*
