Chapter 9 Asynchronous Communication

1. Chapter 9 Asynchronous Communication Using the UART module

2. Simplified UART block diagram figure 19-1 (DS61143)

3. 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 -> 115,384 Baud with an error of just 0.2%, With a value of 78 -> 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 (BREGH=1)

4. UxMODE register register 19-5 (DS61143)

5. UxSTA register register 19-6 (DS61143)

6. Initializing UART2 #include <p32xxxx.h> // 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

7. 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

8. HyperTerminal Setup

9. 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 won’t 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.