Software UART Library

The mikroC PRO for 8051 provides routines for implementing Software UART communication. These routines are hardware independent and can be used with any MCU. The Software UART Library provides easy communication with other devices via the RS232 protocol.

Note: The Software UART library implements time-based activities, so interrupts need to be disabled when using it.

External dependencies of Software UART Library

The following variables must be defined in all projects using Software UART Library:	Description:	Example :
`extern sfr sbit bdata Soft_UART_RX;`	Receive line.	`sbit Soft_UART_RX at P3_0_bit;`
`extern sfr sbit bdata Soft_UART_TX;`	Transmit line.	`sbit Soft_UART_TX at P3_1_bit;`

Soft_UART_Init

Prototype	char Soft_UART_Init(unsigned long baud_rate, char inverted);
Returns	`2` - error, requested baud rate is too low `1` - error, requested baud rate is too high `0` - successful initialization
Description	Configures and initializes the software UART module. Parameters : `baud_rate:` baud rate to be set. Maximum baud rate depends on the MCU’s clock and working conditions. `inverted:` inverted output flag. When set to a non-zero value, inverted logic on output is used. Software UART routines use Delay_Cyc routine. If requested baud rate is too low then calculated parameter for calling `Delay_Cyc` exceeds `Delay_Cyc` argument range. If requested baud rate is too high then rounding error of `Delay_Cyc` argument corrupts Software UART timings.
Requires	Global variables: `Soft_UART_RX` receiver pin `Soft_UART_TX` transmiter pin must be defined before using this function.
Example	This will initialize software UART and establish the communication at 14400 bps: char error; ... error = Soft_UART_Init(14400, 0); // Initialize Soft UART at 14400 bps

Soft_UART_Read

Prototype	char Soft_UART_Read(char * error);
Returns	Byte received via UART.
Description	The function receives a byte via software UART. This is a blocking function call (waits for start bit). Programmer can unblock it by calling Soft_UART_Break routine. Parameters : `error:` Error flag. Error code is returned through this variable. `0` - no error `1` - stop bit error `255` - user abort, Soft_UART_Break called
Requires	Software UART must be initialized before using this function. See the Soft_UART_Init routine.
Example	char data_, error; ... // wait until data is received do data = Soft_UART_Read(&error); while (error); // Now we can work with data: if (data_) {...}

Soft_UART_Write

Prototype	void Soft_UART_Write(char udata);
Returns	Nothing.
Description	This routine sends one byte via the Software UART bus. Parameters : `udata:` data to be sent.
Requires	Software UART must be initialized before using this function. See the Soft_UART_Init routine. Be aware that during transmission, software UART is incapable of receiving data – data transfer protocol must be set in such a way to prevent loss of information.
Example	char some_byte = 0x0A; ... // Write a byte via Soft UART Soft_UART_Write(some_byte);

Soft_UART_Break

Prototype	void Soft_UART_Break();
Returns	Nothing.
Description	Soft_UART_Read is blocking routine and it can block the program flow. Calling this routine from the interrupt will unblock the program execution. This mechanism is similar to WDT. Note: Interrupts should be disabled before using Software UART routines again (see note at the top of this page).
Requires	Nothing.
Example	char data1, error, counter = 0; void Timer1InterruptHandler() org IVT_ADDR_ET1{ if (counter >= 20) { Soft_UART_Break(); counter = 0; // reset counter } else counter++; // increment counter } void main() { TR1_bit = 0; // Stop Timer1 ET1_bit = 1; // Enable Timer1 interrupt TH1 = 0x00; // Set Timer1 high byte TL1 = 0x00; // Set Timer1 low byte TR1_bit = 1; // Run Timer1 EA_bit = 0; // Interrupt disable ... Soft_UART_Init(9600); Soft_UART_Write(0x55); ... // try Soft_UART_Read with blocking prevention mechanism EA_bit = 1; // Interrupt enable data1 = Soft_UART_Read(&error); EA_bit = 0; // Interrupt disable }

Library Example

This example demonstrates simple data exchange via software UART. If MCU is connected to the PC, you can test the example from the mikroC PRO for 8051 USART Terminal Tool.

// Soft UART connections
sbit  Soft_Uart_RX at P3_0_bit;
sbit  Soft_Uart_TX at P3_1_bit;
// End Soft UART connections

char i, error, byte_read;                 // Auxiliary variables

void main(){

  P2 = 0;                                 // P2 is used for error signalization : No error

  error = Soft_UART_Init(4800, 0);        // Initialize Soft UART at 4800 bps

  if (error > 0) {
    P2 = error;                           // Signalize Init error
    while(1) ;                            // Stop program
  }
  Delay_ms(100);

  for (i = 'z'; i >= 'A'; i--) {          // Send bytes from 'z' downto 'A'
    Soft_UART_Write(i);
    Delay_ms(100);
  }
   
  while(1) {                              // Endless loop
    byte_read = Soft_UART_Read(&error);   // Read byte, then test error flag
    if (error)                            // If error was detected
      P2 = error;                         //   signal it on P2
    else
      Soft_UART_Write(byte_read);         // If error was not detected, return byte read
    }      
}

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