RS232.C An Interrupt driven Asyncronous Serial Port

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Eustacia Phyllis Burns
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1 /***************************************************************************/ /* RS232.C An Interrupt driven Asyncronous Serial Port */ /* Date : 06/03/2002 */ /* Purpose : Asyncronous Transmitter & Receiver */ /* Mode : 9600 Baud, 1 stopbit, No parity */ /***************************************************************************/ /************************/ /* Function prototypes */ /************************/ void Setup(void); void TxNextBit(void); void PutChar(char); char GetChar(void); void StartBitDetect(void); void RcvNextBit(void); /* Global Register assignments */ char TxByte; char RxByte; char TxRxBitCount; char SerialStatus; char data; // The Byte to Transmit // The Received Byte // Bit counter for TX/RX // Serial status register // General purpose register /* Flag assignments */ /* Bits of SerialStatus */ #define TxOver 0 // Set when byte transmitted #define TxEnable 1 // Set to enable transmitter #define RxOver 2 // Set when byte received #define RxFrameError 3 // Set when stop bit not zero #define RxStarted 4 // Set when receive character is in progress #define RxBit 5 // Unused. /* Bit Mask definitons */ #define RxInputMask 0x10; // Receiver input pin is connected to RA4, // ie. bit 4 of Port A #define TxOverMask 0x01; #define TxEnableMask 0x02; #define RxOverMask 0x04; #define RxFrameErrorMask 0x08; #define RxStartedMask 0x10; #define RxBitMask 0x20; #define IntconRtcc 0x04; 1

2 /******************************/ /* TxRxBitCount event numbers */ /* ( Transmit mode ) */ /******************************/ #define StartBit 10 #define Bit0 9 #define Bit1 8 #define Bit2 7 #define Bit3 6 #define Bit4 5 #define Bit5 4 #define Bit6 3 #define Bit7 2 #define StopBit 1 /* Port and system assignments */ #define Rs232Out 1 // Rs232 output port bit //#define Baud9600Rate 207 // TMR0 Rate for 4 Mhz clock //#define Baud9600RatePlusHalf 159 // Start bit to first data bit delay (4Mhz) // // *** May need adjustment *** // Very critical for High Baud Rates - // - at low clock speed #define Baud9600Rate 154 #define Baud9600RatePlusHalf 130 #define Rs232BitMask 1 // TMR0 Rate for 8 Mhz clock // Start bit to first data bit delay (8Mhz) // Mask for RS232 send TxNextBit() #define PortAConfig 0x14 // Configure port A ( 1 = input ) // RA4/RTCC = Input / RA2 = Input #define PortBConfig 0xff // Configure port B ( All Input ) #define TxMode 0x40 // Set option reg TMR0 interrupt source from counter. // Max speed prescaler ( 1:2 ) #define RxMode TxMode #define RxStartbitMode 0x78 // TMR0 source is External Clock (falling edge), // Assign prescaler to Watchdog timer // - for detection of START BIT #define POSLOGIC 1 Transmitter ) // When defined then USE TRUE RS232 LEVELS ( For 2

3 /***************************************************************/ /* Start of MAIN */ /***************************************************************/ main() const char Message[] = 'H', 'e', 'l', 'l', 'o', '!', 13, 10, 0x00 ; char a; Setup(); // Set-up the system. set_bit( SerialStatus, TxEnable ); // *** Enable Transmitter *** a = 0; while ( Message[a]!= 0x00 ) PutChar( Message[a++] ); // Send the Message while ( ( SerialStatus & TxOverMask ) == 0 ); while(1) data = GetChar(); // Receive the next Character if ( ( SerialStatus & RxFrameErrorMask ) == 0 ) // Is it a valid character? PutChar(data); // Echo the Received byte transmitted while ( ( SerialStatus & TxOverMask ) == 0 ); // Wait for Character to be // End of Main 3

4 /************************************************************/ /* Process any interrupts */ /************************************************************/ void interrupt( void ) if ( ( INTCON & IntconRtcc )!= 0 ) // If the RTCC times-out // If the transmitter is enabled // - then begin to transmit if ( ( SerialStatus & TxEnableMask )!= 0 ) TxNextBit(); // Send next bit. // If no character is being received else if ( ( SerialStatus & RxStartedMask ) == 0 ) StartBitDetect(); // Initialise Receiver ( wait for a start bit ) else RcvNextBit(); // Else receive the next bit // Return from Interrupt // End of Interrupt Service routine /*******************************/ /* Setup Ports and Interrupts */ /*******************************/ void Setup(void) set_bit( STATUS, RP0 ); OPTION_REG = TxMode; set_tris_a( PortAConfig); set_tris_b( PortBConfig ); // Set Option register to transmitter mode // Setup ports // Set the RS232 output to "Stop bit" output_low_port_a( Rs232Out ); output_high_port_a( Rs232Out ); TxRxBitCount = StartBit; // Setup bit count SerialStatus = 0; // Clear other flags set_bit( SerialStatus, TxOver ); // Set the TxOver status for 'Ready' disable_interrupt( T0IE ); // Disable TMR0 overflow bit enable_interrupt( GIE ); // Enable Global Interrupts 4

5 /********************/ /* Send a character */ /********************/ void PutChar(char ch) // Wait for stop bit of while ( ( SerialStatus & TxOverMask ) == 0 ); // last character to be transmitted clear_bit( SerialStatus, TxOver ); // Reset TxOver flag TxByte = ch; // Setup the data byte to send TMR0 = Baud9600Rate; to - // - initiate transmission # while ( TxRxBitCount!= Bit0 ); // Wait for start bit done. /*******************************************/ /* Send next bit Interrupt service routine */ /*******************************************/ void TxNextBit(void) if ( TxRxBitCount == StartBit ) // Send the start bit output_high_port_a( Rs232Out ); // True RS232 output_low_port_a( Rs232Out ); // Inverted RS232 if ( ( TxRxBitCount <= 9 ) && ( TxRxBitCount >= 2 ) ) // Send the data bits if ( ( TxByte & Rs232BitMask ) == 0 ) // Test bit 0 of data to be sent output_high_port_a( Rs232Out ); // True RS232 output_low_port_a( Rs232Out ); // Inverted RS232 else output_low_port_a( Rs232Out ); // True RS232 output_high_port_a( Rs232Out ); // Inverted RS232 TxByte = ( TxByte >> 1 ); if ( TxRxBitCount == StopBit ) output_low_port_a( Rs232Out ); output_high_port_a( Rs232Out ); // Get next bit to send... // Send the stop bit // True RS232 // Inverted RS232 5

6 TxRxBitCount--; if ( TxRxBitCount == 0 ) TxRxBitCount = StartBit; set_bit( SerialStatus, TxOver ); disable_interrupt( T0IE ); return; // count down the bit counter // Disable TMR0 overflow bit // Return with Interrupt disabled TMR0 = TMR0 + Baud9600Rate; //clear TMR0 overflow flag //enable TMR0 overflow bit // end of TxNextBit(void) /****************************/ /* Start of Receiver code */ /****************************/ /************************************************/ /* Prepare receiver for next character */ /************************************************/ char GetChar(void) SerialStatus = 0; TxRxBitCount = 9; RxByte = 0; set_bit( STATUS, RP0 ); OPTION_REG = RxStartbitMode; TMR0 = 0xff; // Clear Status flags // 8 data bits + 1 stop bit // Clear Rxbyte // Set Option register to receiver mode // Wait for start bit. while( ( SerialStatus & RxOverMask ) == 0 ); // Wait for character ready set_bit( SerialStatus, TxEnable ); set_bit( SerialStatus, TxOver ); TxRxBitCount = StartBit; disable_interrupt( T0IE ); return RxByte; // Enable Transmitter- // -ready for next tx byte // Disable TMR0 overflow bit // Return with the received character // End GetChar() /* Detect the initial start bit of each character */ void StartBitDetect(void) // If RxInput is 0 if ( ( input_port_a() & RxInputMask ) == 0 ) // Start bit detected set_bit( SerialStatus, RxStarted); // Set Rx started flag set_bit( STATUS, RP0 ); // Set Option register to - OPTION_REG = RxMode; // - wait for first data bit TMR0 = Baud9600RatePlusHalf; // Set the timer to wait 1.5 x bit time 6

7 else // False Start bit detected. FalseStartBit: TMR0 = 0xff; set_bit( STATUS, RP0 ); OPTION_REG = RxStartbitMode; clear_bit( SerialStatus, RxStarted ); // Set Option register to receiver mode // end of StartBitDetect() /* Receive the next bit of the current character */ void RcvNextBit(void) if ( TxRxBitCount == 1 ) // Is this the stop bit? if ( ( input_port_a() & RxInputMask ) == 0 ) // Framing error! Stop bit was not '1' set_bit( SerialStatus, RxFrameError ); clear_bit( SerialStatus, RxStarted );// Stop further RTCC interrupts. set_bit( SerialStatus, RxOver ); // Signal GetChar() that byte available disable_interrupt( T0IE ); // Disable TMR0 overflow bit // Receive done else asm rrf _RxByte, F; // if ( ( TxRxBitCount <= 9 ) && ( TxRxBitCount >= 2 ) ) if ( ( input_port_a() & RxInputMask )!= 0 ) set_bit( RxByte, 7 ); else clear_bit( RxByte, 7 ); // Shift RxByte right, for next bit TMR0 = TMR0 + Baud9600Rate; TxRxBitCount--; // Count down bits received. // End of RcvNextBit /* EOF */ 7

Example of Asyncronous Serial Comms on a PIC16F877

/***************************************************************************************/ /* Example of Asyncronous Serial Comms on a PIC16F877 */ /* Target: PIC16F877 */ /* Baud: 9600 */ /* Bits: 8 */