Software RX UART

This implements a software UART for receiving data. It is designed to be small and to run on a microcontroller that either does not have a hardware UART or whose hardware UARTs are all in use for other purposes.

This software has not been tested.

How to Use the Software RX UART

This software UART is implemented in the "SoftRxUart" class.

To use SoftRxUart, you must:

1. Instantiate a SoftRxUart object.
2. Call SoftRxUart::sample() with the state of the digital input pin.

The constructor takes the serial baud rate and interrupt rate as arguments, so you must pass those when instantiating the object. However, as SoftRxUart is designed to be small and simple, the baud rate and interrupt rate both must fit in 8-bit values (less than 256 each). In most cases you will need to scale both values by the same factor; the only thing that matters is that the ratio between the two remains the same.

To read bytes from the SoftRxUart object, call the read method. This method returns the next byte or -1 if the receive queue is empty.

That's it! Note that the SoftRxUart class does not manage the timer interrupts or input pin itself. Those must be set up and handled separately.

Example

Let's look at an example of using SoftRxUart on an AVR microcontroller. The exact details on how to create an interrupt handler and read a digital input pin depend on the particular microcontroller and on your application's needs (as well as your compiler's brand), but this should at least illustrate how to use SoftRxUart.

Say you are using Timer 1, running at 16000Hz, and reading from pin PD4. You want to read at 2400 baud. Your code will look similar to the following:

#define BAUD_RATE     2400
#define INT_RATE     16000
#define INT_BAUD_GCD   100

SoftRxUart mySoftRxUart(BAUD_RATE/INT_BAUD_GCD, INT_RATE/INT_BAUD_GCD);

ISR(TIMER1_COMPA_vect)
{
    mySoftRxUart.sample(PIND & PD4);
}

The macros are not required, but they show what is happening more clearly. As mentioned above, the SoftRxUart constructor takes the baud and interrupt rates and that they must be limited to 8-bit values. The INT_BAUD_GCD macro is used to scale the baud rate and interrupt rate to fit them in 8 bits while maintaining the ratio between them. In this example, the baud rate and interrupt rate are scaled to 24 and 160, respectively.

Limitations

SoftRxUart implements only 8-bit, no parity, and 1 stop bit. Technically, it can support more than 1 stop bit (as a stop bit is simply a gap between frames), and it technically could support 7-bit with even/odd/mark/space parity while leaving it up to the application to handle the upper bit in the received byte.

The baud rate can be virtually anything as long as the timer interrupt runs at least twice as fast as the baud, though ideally the interrupt should run at least four times as fast as the baud rate. The baud rate also cannot be less than 1/255 of the interrupt rate.

License

Copyright 2017 Christopher Williams.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

3. Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.