Chapter%209%20Asynchronous%20Communication

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Chapter 9 Asynchronous Communication

• Using the UART module

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Simplified UART block diagram
figure 19-1 (DS61143)
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Baud Rate setting
In our case this translates to the following
expression U2BREG (36,000,000 / 4 / 115,200)
-1 77.125 To decide how to best round out the
result, use the reverse formula to calculate the
actual baud-rate and determine the percentage
error Error ((Fpb / 4 / (U2BREG 1)) baud
rate) / baud rate With a value of 77 -gt
115,384 Baud with an error of just 0.2, With a
value of 78 -gt 113,924 baud, 1.1 error, Both
are within the acceptable tolerance range for a
standard RS232 port (/- 2) . We can therefore
define the constant BRATE as define BRATE
77 // 115,200 Bd (BREGH1)
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UxMODE register
register 19-5 (DS61143)
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UxSTA register
register 19-6 (DS61143)
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Initializing UART2
  include ltp32xxxx.hgt   // I/O definitions for
the Explorer16 define CTS _RF12
// Clear To Send, input define RTS _RF13
// Request To Send, output define
TRTS TRISFbits.TRISF13 // Tris control for
RTS pin   void initU2( void) U2BRG
BRATE // initialize the baud rate generator
U2MODE U_ENABLE // initialize the UART
module U2STA U_TX // enable the
Transmitter TRTS 0 // make RTS an
output pin RTS 1 // set RTS
default status (not ready) // initU2
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Sending and Receiving Data
int putU2( int c) while ( CTS)
// wait for !CTS, clear to send while (
U2STAbits.UTXBF) // wait while Tx buffer full
U2TXREG c return c // putU2 char
getU2( void) RTS 0 //
assert Request To Send !RTS while (
!U2STAbits.URXDA) // wait for a new char to
arrive RTS 1 return U2RXREG
// read char from receive buffer // getU2
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HyperTerminal Setup
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Tips and Tricks

• To re-direct the output stream of the standard C
  library (stdio.h) functions such as printf() to a
  UART
• Define the function _mon_putc()
• Note that a weak definition is already provided
  in the library to send the default output stream
  (stdout) to UART2 (convenient for all Explorer16
  users).
• Similarly define _mon_getc()
• A default weak version is already provided in
  the library as well, connecting UART2 receiver to
  the input stream (stdin).
• Weak means that the compiler wont complain when
  you define a new function with the same name, it
  will simply replace it with the new one you
  provide.
• NOTE
• You are responsible for the UART initialization!
• Before the first call to any stdio function
  (printf()) make sure the UART2 is enabled and
  the baud rate is set correctly.