Example of Asyncronous Serial Comms on a PIC16F877

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Scott Parrish
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1 /***************************************************************************************/ /* Example of Asyncronous Serial Comms on a PIC16F877 */ /* Target: PIC16F877 */ /* Baud: 9600 */ /* Bits: 8 */ /* Parity: none */ /* Stop bits : 1 */ /* Warning: Clock frequency dependant */ /* This example sends a message then */ /* echos lines typed on a terminal. */ /***************************************************************************************/ #pragma CLOCK_FREQ // Processor clock frequency in Hz. // Default value for PIC is (4MHz) // and for SX (50MHz). // This will not affect the comms speed! // if you use a different frequency crystal then change SPBRG // My PORT Configuration #define PortAConfig 0x00 #define PortBConfig 0xf0 #define PortCConfig 0x98 /* SCL & SDA as Inputs */ #define PortDConfig 0x00 #define PortEConfig 0x00 // USART Register bits #define CSCR 7 #define TX9 6 #define TXEN 5 #define SYNC 4 #define BRGH 2 #define TRMT 1 #define TX9D 0 #define SPEN 7 #define RX9 6 #define SREN 5 #define CREN 4 #define ADDEN 3 #define FERR 2 #define OERR 1 #define RX9D 0 #define TRMT_MASK 2 // Masks for PIR1 #define PSPIF_MASK 0x80 #define ADIF_MASK 0x40 #define RCIF_MASK 0x20 #define TXIF_MASK 0x10 // Registers for I2C char SSPSTAT@0x94; // Bank 1 char SSPCON@0x14; // Bank 0 char SSPCON2@0x91; // Bank 1 char SSPBUF@0x13; // I2C Buffer char SSPADD@0x93; // I2C Slave Address register 1

2 // Bits of SSPSTAT #define SMP 7 #define CKE 6 #define D_A 5 #define P 4 #define S 3 #define R_W 2 #define R_W_MASK 0x04 #define UA 1 #define BF 0 // Bits of SSPCON2 #define GCEN 7 #define ACKSTAT 6 #define ACKDT 5 #define ACKEN 4 #define RCEN 3 #define PEN 2 #define RSEN 1 #define SEN 0 // Bits of PIR1 #define PSPIF 7 #define ADIF 6 #define RCIF 5 #define TXIF 4 #define SSPIF 3 #define SSPIF_MASK 0x08 #define CCP1IF 2 #define TMR2IF 1 #define TMR1IF 0 // Bits of SSPCON #define WCOL 7 #define SSPOV 6 #define SSPEN 5 #define CKP 4 #define SSPM3 3 #define SSPM2 2 #define SSPM1 1 #define SSPM0 0 // Port addresses char PORTC@0x07; char PORTD@0x08; char PORTE@0x09; // USART Registers char TXREG@0x19; char RCREG@0x1a; char TXSTA@0x98; char RCSTA@0x18; char SPBRG@0x99; // Extra Ports on PIC16F877 char TRISC@0x87; char TRISD@0x88; char TRISE@0x89; 2

3 // Other regs char char char char char // ADC bits char char // Function Declarations void Setup(void); void ConfigureComms(void); void SendChar(char); char RxChars(void); // Setup the PIC // Configure the comms // Send a character // Receive a characters when RX Interrupt occurs void SendString( const char *ptr );// Send a const string char MyFlags; char BufferIndex; char RxFifo[20]; // Status flags // Yep! an index to the buffer // Receive data buffer. #define RX_BUFFER_SIZE 20 #define BufferReady 0 #define BufferReadyMask 0x01 // Bit 0 of MyFlags const char *Msg1 ="PIC16F877 RS232 Test"; 3

4 // // Start of MAIN // void main(void) Setup(); /* Setup the PIC */ BufferIndex = 0; clear_bit( MyFlags, BufferReady ); /* Clear the buffer ready flag */ SendString( Msg1 ); /* Send a message to the terminal */ while( 1 ) if ( MyFlags & BufferReadyMask ) /* ECHO the buffer back to the terminal */ /* After the enter key was pressed */ while( RxFifo[ BufferIndex ]!= 0 ) SendChar( RxFifo[ BufferIndex++ ] ); BufferIndex = 0; clear_bit( MyFlags, BufferReady ); /* Clear the buffer ready flag */ // end while 1 // end of Main() void interrupt(void) if ( ( PIR1 & RCIF_MASK )!= 0 ) // If USART RX Interrupt RxChars(); // Process the received character clear_bit( PIR1, RCIF ); // Clear flag // Return from Interrupt 4

5 /* Send a const string */ /* ( Null terminated ) */ void SendString(const char *ptr) char i; i = 0; // Check for end of string while( ptr[i]!= 0 ) SendChar( ptr[i++] ); /* setup PIC16F877 options,ports,interrupts */ void Setup(void) INTCON = 0x00; set_bit( INTCON, GIE ); set_bit( INTCON, PEIE ); // Enable Global Interrupts // Enable all Peripheral Interrupts set_bit( STATUS, RP0 ); // Register page 1 // Compiler doesn't detect ram bank switch here! // with OPTION_REG OPTION_REG = 0x0C; // Set Option register // Prescaler = WDT // WDT rate := 1:16 TRISD = PortDConfig; ADCON1 = 0x7f; // Disable ADC TRISA = PortAConfig; TRISB = PortBConfig; TRISC = PortCConfig; TRISE = PortEConfig; clear_bit( STATUS, RP0 ); // Register page 0 PIR1 = 0; ConfigureComms(); /* Configure USART for Asyncronous Comms */ 5

6 /*******************************************************/ /* Configure USART for communications */ /* Asynchronous mode */ /* 19,200 Baud ( With Mhz Clock ) */ /* 8 data bits ( For other rates see PIC16F8XX Data ) */ /* 2 stop bits */ /* No Parity */ /*******************************************************/ void ConfigureComms(void) set_bit( RCSTA, SPEN ); // Enable Serial port clear_bit( RCSTA, RX9 ); // 8 bit receive mode set_bit( STATUS, RP0 ); // *** Register page 1 *** clear_bit( TXSTA, TX9 ); // 8 bit transmit mode // SPBRG = 0; // SPBRG = 1 ( Set Baud rate 115,200 ) // SPBRG = 5; // SPBRG = 5 ( Set Baud rate 38,400 ) // SPBRG = 22; // SPBRG = 22 ( Set Baud rate 9,600 ) // SPBRG = 11; // SPBRG = 11 ( Set Baud rate 19,200 ) SPBRG = 22; // SPBRG = 22 ( Set Baud rate 9,600 ) // // For this value at a given clock rate // see the microchip document // set_bit( TXSTA, BRGH ); // RRGH = 1 ( High speed mode ) clear_bit( TXSTA, SYNC ); // Asyncronous mode; set_bit( TXSTA, TXEN ); set_bit( PIE1, RCIE ); // Enable Transmitter // Enable Receive Interrupt clear_bit( STATUS, RP0 ); // *** Register page 0 *** set_bit( RCSTA, CREN ); clear_bit( PIR1, RCIF ); set_bit( INTCON, PEIE ); set_bit( INTCON, GIE ); // Enable continuous receive // Clear Receive Interrupt flag // Enable all Peripheral Interrupts // Enable Global Interrupts 6

7 /* Send a character over the RS232 Port */ void SendChar(char ch) char TxEmpty; asm bsf STATUS, RP0; // *** Register page 1 *** asm movf TXSTA, W; // Save TXSTA value asm bcf STATUS, RP0; // *** Register page 0 *** asm movwf _TxEmpty_SendChar; // Restore TXSTA value while ( ( TxEmpty & TRMT_MASK ) == 0 ) // Wait for TX Empty asm bsf STATUS, RP0; // *** Register page 1 *** asm movf TXSTA, W; // Save TXSTA value asm bcf STATUS, RP0; // *** Register page 0 *** asm movwf _TxEmpty_SendChar; // Restore TXSTA value asm bcf STATUS, RP0; // *** Register page 0 *** TXREG = ch; // Load the TXREG /* Receive a character over the RS232 Port */ /* Called from Interrupt service routine */ /* Returns the char received */ /* and saves it in the buffer */ char RxChars(void) if ( ( RCSTA & 6 ) == 0 ) // Then if no errors // Process received character // If terminated by a carrage return // or Buffer end reached received */ if ( ( RCREG == 13 ) ( BufferIndex == RX_BUFFER_SIZE ) ) set_bit( MyFlags, BufferReady ); /* Set a flag to indicate line else RxFifo[ BufferIndex ] = 0; /* NULL Terminate the buffer */ RxFifo[ BufferIndex++] = RCREG; // Save the data set_bit( RCSTA, CREN ); // Enable receiver. else // process any errors here // Beware, we are in the Interrupt routine. //... clear_bit( RCSTA, CREN ); // Clear any errors set_bit( RCSTA, CREN ); // Enable receiver. return RCREG; 7

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