Arduino Hardware Serial with RTS/CTS Flow Control

An Arduino AVR library to add hardware flow control via RTS/CTS to the serial ports. It’s based on the original HardwareSerial files modified to implement the flow control. The library is backward compatible, so it’s possible to compile old projects without any changes.

How to install

• Click on the "Clone or download" button in the upper right corner.
• Extract the ZIP file.
• Locate the original HardwareSerial .cpp and .h files on your disk drive: HardwareSerial.cpp HardwareSerial0.cpp HardwareSerial1.cpp HardwareSerial2.cpp HardwareSerial3.cpp HardwareSerial.h HardwareSerial_private.h
• Replace the original files with the new version (make a backup copy of the original files for safety)

Getting started

Wiring

Connect the 4 wires as described in the figure:

NOTE:

CTS must be connected to an interrupt pin (marked as INTx). If connected to a PCINTx pin it will not work. See Arduino external interrupt for reference. The library doesn’t check if the CTS pin is an INT pin, so be sure to use the right one.

RTS can be connected to any free pin.

Software

The begin() function has been expanded to support the RTS and CTS pin definition.

The old, and still supported, way to call begin() without flow control is:

begin(115200);

or

begin(115200, SERIAL_8N1);

Now, to use the flow control, it’s possible to call it with the new additional parameters:

begin(115200, 3, 4);   // Speed, CTS pin, RTS pin);

or

begin(115200, SERIAL_8N1, 3, 4);   // Speed, mode, CTS pin, RTS pin);

If the communication has to work in one direction only (a device only sends, and the other only receives), it’s possible to use -1 as CTS or RTS pin. In this way the pin will not be assigned and it’s not necessary to wire it. Example:

begin(115200, SERIAL_8N1, 3, -1);   // Speed, mode, CTS pin, no RTS);

Examples

To test the library, two ATmega2560 were connected together as described in the Wiring section, and two test programs were created (both present in the examples folder): test-rx and test-tx. The board with the test-rx program sends some info to the IDE monitor console. It shows the receiving progress and it checks if the received data are correct or if characters were lost. test-tx just sends a long train of characters to the specified serial using the flow control. It’s important to launch the RX program before the TX program. To restart the test, just reset the RX program and then the TX program using the reset button.

Acknowledgements

The library is based on the original Arduino AVR Core HardwareSerial library.