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8051 Serial Communication Tutorial (UART)



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 8051  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 8051  can understand. It also boosts the voltage of our 8051  to the needed +/-12V of the RS232 protocol so that a computer can understand our 8051  and vice versa. To get our 8051  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.




UART Library
The UART hardware module is available with a number of 8051 compliant MCUs. The mikroC PRO for 8051 UART Library provides comfortable work with the Asynchronous (full duplex) mode.
Library Routines
Notes:
  • UART routines 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 2.
    Number of UART modules per MCU differs from chip to chip. Please, read the appropriate datasheet before utilizing this library.
Example: UART2_Init(9600); initializes UART 2 module at 9600 bps.
  • Some 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).
  • Some of the MCUs do not support UARTx_Init_Advanced routine. Please, refer to the appropriate datasheet.
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
  • frame size 8 bits
  • 1 STOP bit
  • parity mode disabled
  • disabled automatic address recognition
Parameters :
  • baud_rate: requested baud rate
Refer to the device datasheet for baud rates allowed for specific Fosc.
Requires
MCU with the UART module.
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 adv_setting);
Returns
Nothing.
Description
Configures and initializes UART module.
Parameters :
  • baud_rate sets the desired UART baud rate
  • adv_setting: UART module configuration flags. Predefined library constants (see the table below) can be ORed to form appropriate configuration value.
Description
Predefined library const
Parity constants:
Parity mode disabled
_UART_NOPARITY
Even parity
_UART_EVENPARITY
Odd parity
_UART_ODDPARITY
Mark parity
_UART_MARKPARITY
Space parity
_UART_SPACEPARITY
Stop bit constants:
1 stop bit
_UART_ONE_STOPBIT
2 stop bits
_UART_TWO_STOPBITS
Output mode constants:
Output set as quasi-bidirectional (8051)
_UART_OUTPUT_8051
Output set as push-pull
_UART_OUTPUT_PUSH_PULL
Output set as open-drain
_UART_OUTPUT_OPEN_DRAIN
Notes:
  • Some MCUs do not support advanced configuration of the UART module. Please consult appropriate datasheet.
  • Advanced parity and stop bit settings are supported by some Silicon Laboratories MCU's, while output settings by some ATMEL MCU's. Please, consult appropriate datasheet before using UARTx_Init_Advanced routine.
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
MCU with the UART module.
The UART module must be initialized before using this routine. See UARTx_Init and UARTx_Init_Advanced routines.
Example
char receive;
...
// read data if ready
if (UART1_Data_Ready())
  receive = UART1_Read();
UARTx_Read
Prototype
char UARTx_Read();
Returns
Returns the received byte.
Description
The function receives a byte via UART. Use the UARTx_Data_Ready function to test if data is ready first.
Requires
MCU with the UART module.
The UART module must be initialized before using this routine. See UARTx_Init and UARTx_Init_Advanced routines.
Example
char receive;
...
// read data if ready
if (UART1_Data_Ready())
  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 and UARTx_Init_Advanced routines.
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 and UARTx_Init_Advanced routines.
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 and UARTx_Init_Advanced routines.
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)())
Returns
Nothing.
Description
Sets active UART module which will be used by the UART library routines.
Parameters :
Requires
Routine is available only for MCUs with two UART modules.
Used UART module must be initialized before using this routine. See UARTx_Init and UARTx_Init_Advanced routines.
Example
// Activate UART2 module
UART_Set_Active(&UART2_Read, &UART2_Write, &UART2_Data_Ready);
Library Example
This example demonstrates simple data exchange via UART. If MCU is connected to the PC, you can test the example from the mikroC PRO for 8051 USART Terminal.
char uart_rd;

void main() {
 
  UART1_Init(4800);               // Initialize UART module at 4800 bps
  Delay_ms(100);                  // Wait for UART module to stabilize
 
  UART1_Write_Text("Start");
  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
    }
  }
}








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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.
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