Software UART Library

The mikroPascal 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 :
`var Soft_UART_RX: sbit; sfr; bdata; external;`	Receive line.	`var Soft_UART_RX: sbit at P3_0_bit;`
`var Soft_UART_TX: sbit; sfr; bdata; external;`	Transmit line.	`var Soft_UART_TX: sbit at P3_1_bit;`

Soft_UART_Init

Prototype	function Soft_UART_Init(baud_rate: dword; inverted: byte): word;
Returns	`2` - error, requested baud rate is too low `1` - error, requested baud rate is too high `0` - successfull 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	var Soft_UART_TX: sbit at P3_1_bit; var Soft_UART_RX: sbit at P3_0_bit; var error : byte; ... // Initialize Software UART communication on pins Rx, Tx, at 9600 bps error := Soft_UART_Init(9600, 0);

Soft_UART_Read

Prototype	function Soft_UART_Read(var error: byte): byte;
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	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 Timer1InterruptHandler(); org IVT_ADDR_ET1; begin counter := 0; if (counter >= 20) then begin Soft_UART_Break(); counter := 0; // reset counter end else Inc(counter); // increment counter end; begin 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 ... 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 8051 USART Terminal Tool.

program Soft_UART;

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

var i, error, byte_read : byte;               // Auxiliary variables
begin
   Soft_UART_Init(4800, 0);                  // Initialize Soft UART at 4800 bps
     for i := 48 to 122 do                   // Send bytes from '0' downto '9'
      begin
        Soft_UART_Write(i);
        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
        P0 := 0xAA                           //   signal it on PORT0
      else
        Soft_UART_Write(byte_read);          // If error was not detected, return byte read
    end;
  end.

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