First, a quick history of RS232. What is RS232? It's just a name for a standard that has propagated from generation to generation of computers. The first computers had serial ports that used RS232, and even current computers have serial ports (or at least USB ports that act like RS232 ports). Back in the day, serial information needed to be passed from devices like printers, joysticks, scanners, etc to the computer. The simplest way to do this was to pass a series of 1s and 0s to the computer. Both the computer and the device agreed on a speed of information - 'bits per second'. A computer would pass image data to a printer at 9600 bits per second and the printer would listen for this stream of 1s and 0s expecting a new bit every 1/9600 = 104us (104 micro-seconds, 0.000104 seconds). As long as the computer output bits at the pre-determined speed, the printer could listen.
Zoom forward to today. Electronics have changed a bit. Before they were relatively high power, high voltage devices. The standard that is 'RS232' dictates that a bit ranges from -12V to +12V. Modern electronics do not operate at such high positive and negative voltages. In fact, our AVR  runs 0V to 5V. So how do we get our 5V micro to talk the RS232 +/-12V voltages? This problem has been solved by the IC manufacturers of the world. They have made an IC that is generically known as the MAX232 (very close to RS232, no?).
The MAX232 is an IC originally designed by a company called Maxim IC that converts the +/-12V signals of RS232 down to the 0/5V signals that our AVR  can understand. It also boosts the voltage of our AVR  to the needed +/-12V of the RS232 protocol so that a computer can understand our AVR  and vice versa. To get our AVR  IC sending serial characters to a computer, we have to send these serial signals through a MAX232 circuit so that the computer receives +/-12V RS232 signals. Don't worry if you're working with a chip labeled 'ICL232' or 'ST232' - these are just generics of the MAX232. Everyone says 'MAX232'. The ICs all function the same and nearly all have the same pinout. 

Schematic Diagram 

UART Library

The UART hardware module is available with a number of AVR MCUs. mikroC PRO for AVR UART Library provides comfortable work with the Asynchronous (full duplex) mode.

You can easily communicate with other devices via RS-232 protocol (for example with PC, see the figure at the end of the topic – RS-232 HW connection). You need a AVR MCU with hardware integrated UART, for example ATmega16. Then, simply use the functions listed below.

  Important :

• AVR MCUs require you to specify the module you want to use. To select the desired UART, simply change the letter x in the prototype for a number from 1 to 4. Number of UART modules per MCU differs from chip to chip. Please, read the appropriate datasheet before utilizing this library.
• For the XMEGA family of MCUs change the X in the routine prototype with C0, C1, D0, D1, E0, E1, F0 or F1 (MCU dependent).
• Some of the AVR MCUs do not support UARTx_Init_Advanced routine. Please, refer to the appropriate datasheet.
• Some AVR MCUs have multiple UART modules. Switching between the UART modules in the UART library is done by the UART_Set_Active function (UART module has to be previously initialized).

Library Routines

• UARTx_Init
• UARTx_Init_Advanced
• UARTx_Data_Ready
• UARTx_Tx_Idle
• UARTx_Read
• UARTx_Read_Text
• UARTx_Write
• UARTx_Write_Text
• UART_Set_Active

Generic Routines

• UART_Data_Ready
• UART_Tx_Idle
• UART_Read
• UART_Read_Text
• UART_Write
• UART_Write_Text

UARTx_Init

Prototype	void UARTx_Init(unsigned long baud_rate);
Returns	Nothing.
Description	Configures and initializes the UART module. The internal UART module module is set to : receiver enabled transmitter enabled frame size 8 bits 1 STOP bit parity mode disabled asynchronous operation Parameters : baud_rate: requested baud rate Refer to the device data sheet for baud rates allowed for specific Fosc.   Note : Calculation of the UART baud rate value is carried out by the compiler, as it would produce a relatively large code if performed on the libary level. Therefore, compiler needs to know the value of the parameter in the compile time. That is why this parameter needs to be a constant, and not a variable. Also, UART baud rate it is calculated under assumption that peripheral clock is equal to the system clock.
Requires	You'll need AVR MCU with hardware UART. UARTx_Init needs to be called before using other functions from UART Library.
Example	// Initialize hardware UART1 and establish communication at 9600 bps UART1_Init(9600);

UARTx_Init_Advanced

Prototype	void UARTx_Init_Advanced(unsigned long baud_rate, char parity, char stop_bits);
Returns	Nothing.
Description	Configures and initializes UART module. Parameter baud_rate configures UART module to work on a requested baud rate. Parameters parity and stop_bits determine the work mode for UART, and can have the following values: Description Predefined library const Parity constants: Parity mode disabled _UART_NOPARITY Even parity _UART_EVENPARITY Odd parity _UART_ODDPARITY Stop bit constants: 1 stop bit _UART_ONE_STOPBIT 2 stop bits _UART_TWO_STOPBITS   Note : Some MCUs do not support advanced configuration of the UART module. Please consult appropriate daatsheet. Calculation of the UART baud rate value is carried out by the compiler, as it would produce a relatively large code if performed on the libary level. Therefore, compiler needs to know the value of the parameter in the compile time. That is why this parameter needs to be a constant, and not a variable. Also, UART baud rate it is calculated under assumption that peripheral clock is equal to the system clock.
Requires	MCU must have UART module.
Example	// Initialize hardware UART1 module and establish communication at 9600 bps, 8-bit data, even parity and 2 STOP bits UART1_Init_Advanced(9600, _UART_EVENPARITY, _UART_TWO_STOPBITS);

UARTx_Data_Ready

Prototype	char UARTx_Data_Ready();
Returns	1 if data is ready for reading 0 if there is no data in the receive register
Description	Use the function to test if data in receive buffer is ready for reading.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	// If data is ready, read it: if (UART1_Data_Ready() == 1) {  receive = UART1_Read();  }

UARTx_Tx_Idle

Prototype	char UARTx_Tx_Idle();
Returns	1 if the data has been transmitted 0 otherwise
Description	Use the function to test if the transmit shift register is empty or not.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	// If the previous data has been shifted out, send next data: if (UART1_Tx_Idle() == 1) {   UART1_Write(_data);  }

UARTx_Read

Prototype	char UARTx_Read();
Returns	Returns the received byte.
Description	Function receives a byte via UART. Use the UARTx_Data_Ready function to test if data is ready first.
Requires	The UART module must be initialized before using this routine. See UARTx_Init routine.
Example	// If data is ready, read it: if (UART1_Data_Ready() == 1) {  receive = UART1_Read();  }

UARTx_Read_Text

Prototype	void UARTx_Read_Text(char *Output, char *Delimiter, char Attempts);
Returns	Nothing.
Description	Reads characters received via UART until the delimiter sequence is detected. The read sequence is stored in the parameter output; delimiter sequence is stored in the parameter delimiter. This is a blocking call: the delimiter sequence is expected, otherwise the procedure exits (if the delimiter is not found). Parameters : Output: received text Delimiter: sequence of characters that identifies the end of a received string Attempts: defines number of received characters in which Delimiter sequence is expected. If Attempts is set to 255, this routine will continuously try to detect the Delimiter sequence.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	Read text until the sequence “OK” is received, and send back what’s been received: UART1_Init(4800);                         // initialize UART1 module Delay_ms(100); while (1) {   if (UART1_Data_Ready() == 1) {          // if data is received     UART1_Read_Text(output, "OK", 10);    // reads text until 'OK' is found     UART1_Write_Text(output);             // sends back text  } }

UARTx_Write

Prototype	void UARTx_Write(char data_);
Returns	Nothing.
Description	The function transmits a byte via the UART module. Parameters : data_: data to be sent
Requires	MCU with the UART module. The UART module must be initialized before using this routine. See UARTx_Init routine.
Example	unsigned char data_ = 0x1E; ... UART1_Write(data_);

UARTx_Write_Text

Prototype	void UARTx_Write_Text(char * UART_text);
Returns	Nothing.
Description	Sends text via UART. Text should be zero terminated. Parameters : UART_text: text to be sent
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	Read text until the sequence “OK” is received, and send back what’s been received: UART1_Init(4800);                         // initialize UART1 module Delay_ms(100); while (1) {   if (UART1_Data_Ready() == 1) {          // if data is received     UART1_Read_Text(output, "OK", 10);    // reads text until 'OK' is found     UART1_Write_Text(output);             // sends back text  } }

UART_Set_Active

Prototype	void UART_Set_Active(char (*read_ptr)(), void (*write_ptr)(unsigned char data_), char (ready_ptr)(), char (*tx_idle_ptr)())
Returns	Nothing.
Description	Sets active UART module which will be used by the UART library routines. Parameters : read_ptr: UARTx_Read handler write_ptr: UARTx_Write handler ready_ptr: UARTx_Data_Ready handler tx_idle_ptr: UARTx_Tx_Idle handler
Requires	Routine is available only for MCUs with multiple UART modules. Used UART module must be initialized before using this routine. See UARTx_Init routine
Example	UART1_Init(9600);                    // initialize UART1 module UART2_Init(9600);                    // initialize UART2 module RS485Master_Init();                  // initialize MCU as Master UART_Set_Active(&UART1_Read, &UART1_Write, &UART1_Data_Ready, &UART1_Tx_Idle); // set UART1 active RS485Master_Send(dat,1,160);        // send message through UART1 UART_Set_Active(&UART2_Read, &UART2_Write, &UART2_Data_Ready, &UART2_Tx_Idle); // set UART2 active RS485Master_Send(dat,1,160);        // send through UART2

UART_Data_Ready

Prototype	char UART_Data_Ready();
Returns	1 if data is ready for reading 0 if there is no data in the receive register
Description	Use the function to test if data in receive buffer is ready for reading. This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	// If data is ready, read it: if (UART_Data_Ready() == 1) {  receive = UART_Read();  }

UART_Tx_Idle

Prototype	char UART_Tx_Idle();
Returns	1 if the data has been transmitted 0 otherwise
Description	Use the function to test if the transmit shift register is empty or not. This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	// If the previous data has been shifted out, send next data: if (UART_Tx_Idle() == 1) {   UART_Write(_data);  }

UART_Read

Prototype	char UART_Read();
Returns	Returns the received byte. This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Description	Function receives a byte via UART. Use the UART_Data_Ready function to test if data is ready first.
Requires	The UART module must be initialized before using this routine. See UARTx_Init routine.
Example	// If data is ready, read it: if (UART_Data_Ready() == 1) {  receive = UART_Read();  }

UART_Read_Text

Prototype	void UART_Read_Text(char *Output, char *Delimiter, char Attempts);
Returns	Nothing.
Description	Reads characters received via UART until the delimiter sequence is detected. The read sequence is stored in the parameter output; delimiter sequence is stored in the parameter delimiter. This is a blocking call: the delimiter sequence is expected, otherwise the procedure exits (if the delimiter is not found). Parameters : Output: received text Delimiter: sequence of characters that identifies the end of a received string Attempts: defines number of received characters in which Delimiter sequence is expected. If Attempts is set to 255, this routine will continuously try to detect the Delimiter sequence. This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	Read text until the sequence “OK” is received, and send back what’s been received: UART_Init(4800);                         // initialize UART1 module Delay_ms(100); while (1) {   if (UART_Data_Ready() == 1) {          // if data is received     UART_Read_Text(output, "OK", 10);    // reads text until 'OK' is found     UART_Write_Text(output);             // sends back text  } }

UART_Write

Prototype	void UART_Write(char data_);
Returns	Nothing.
Description	The function transmits a byte via the UART module. Parameters : data_: data to be sent This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Requires	MCU with the UART module. The UART module must be initialized before using this routine. See UARTx_Init routine.
Example	unsigned char data_ = 0x1E; ... UART_Write(data_);

UART_Write_Text

Prototype	void UART_Write_Text(char * UART_text);
Returns	Nothing.
Description	Sends text via UART. Text should be zero terminated. Parameters : UART_text: text to be sent This is a generic routine which uses the active UART module previously activated by the UART_Set_Active routine.
Requires	UART HW module must be initialized and communication established before using this function. See UARTx_Init.
Example	Read text until the sequence “OK” is received, and send back what’s been received: UART_Init(4800);                         // initialize UART1 module Delay_ms(100); while (1) {   if (UART_Data_Ready() == 1) {          // if data is received     UART_Read_Text(output, "OK", 10);    // reads text until 'OK' is found     UART_Write_Text(output);             // sends back text  } }

Code

The tutorial demonstrates a simple data exchange via UART. When AVR MCU receives data, it immediately sends it back. If AVR is connected to the PC (see the figure below), you can test the example from the mikroC PRO for AVR terminal for RS-232 communication, menu choice Tools › Terminal.

char uart_rd;

void main() {

  UART1_Init(9600);               // Initialize UART module at 9600 bps

  Delay_ms(100);                  // Wait for UART module to stabilize

  UART1_Write_Text("Init");

  UART1_Write(13);UART1_Write(10);

  while (1) {                     // Endless loop

   if (UART1_Data_Ready()) {      // If data is received,

     uart_rd = UART1_Read();      // read the received data,

     UART1_Write(uart_rd);        // and send data via UART

    }

  }

}

}