Software UART Library

The mikroPascal PRO for AVR 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.

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

Soft_UART_Init

Prototype	function Soft_UART_Init(var port : byte; rx_pin, tx_pin, baud_rate : dword, inverted : byte): byte;
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 : `port:` port to be used. `rx_pin:` sets rx_pin to be used. `tx_pin:` sets tx_pin to be used. `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	Nothing.
Example	var error : byte; ... // Initialize Software UART communication on pins Rx, Tx, at 14400 bps error := Soft_UART_Init(PORTC, 7, 6, 14400, 0);

Soft_UART_Read

Prototype	function Soft_UART_Read(var error: byte): byte;
Returns	Returns a received byte.
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. Values : `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	Here’s a loop which holds until data is received: var data : byte; error : byte; ... // wait until data is received repeat data_ := Soft_UART_Read(error); until (error=0); // Now we can work with data: if ( data ) then begin ... end

Soft_UART_Write

Prototype	procedure Soft_UART_Write(udata: byte);
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	var some_byte : byte; ... // Write a byte via Soft UART some_byte := 0x0A; Soft_UART_Write(some_byte);

Soft_UART_Break

Prototype	procedure Soft_UART_Break();
Returns	Nothing.
Description	Soft_UART_Read is blocking routine and it can block the program flow. Call this routine from interrupt to 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	var data1, error, counter : byte ; procedure Timer0Overflow_ISR(); org 0x12; begin counter := 0; if (counter >= 20) then begin Soft_UART_Break(); counter := 0; // reset counter end else Inc(counter); // increment counter end; begin TOIE0_bit := 1; // Timer0 overflow interrupt enable TCCR0_bit := 5; // Start timer with 1024 prescaler SREG_I_bit := 0; // Interrupt disable ... Soft_UART_Init(9600); Soft_UART_Write(0x55); ... // try Soft_UART_Read with blocking prevention mechanism SREG_I_bit := 1; // Interrupt enable data1 := Soft_UART_Read(&error); SREG_I_bit := 0; // Interrupt disable ... end;

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 mikroPascal PRO for AVR USART Terminal Tool.

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program Soft_UART;

var error : byte;
    counter, byte_read : byte;                     // Auxiliary variables

begin

  DDRB := 0xFF;                                    // Set PORTB as output (error signalization)
  PORTB := 0;                                      // No error
  
  error := Soft_UART_Init(PORTD,0,1,9600,0);      // Initialize Soft UART at 9600 bps
  if (error > 0) then
    begin
      PORTB := error;                              // Signalize Init error
      while (TRUE) do nop;                         // Stop program
    end;
  Delay_ms(100);

  for counter := 'z' downto 'A' do                 // Send bytes from 'z' downto 'A'
  begin
    Soft_UART_Write(counter);
    Delay_ms(100);
  end;

  while TRUE do                                    // Endless loop
    begin
      byte_read := Soft_UART_Read(error);          // Read byte, then test error flag
      if (error <> 0) then                         // If error was detected
        PORTB := error                             //   signal it on PORTB
      else
        Soft_UART_Write(byte_read);                // If error was not detected, return byte read
    end;
end.

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