Finally I’ve managed to get some time to implement hardware flow control in UsbSerial! It is probably the most trickiest part of treating with these chipsets because every single one handles this in a very particular manner.

– CP210x devices check the modem status through a usb control transfer, so I was forced to perform a polling in a new thread every X time to check for changes.
– FTDI devices has a more clever way. Every 40 ms a 2-byte package is sent to the host which contains information about the modem signals and the flag errors.
– PL2303 (not implemented yet) have a USB INT endpoint which it is the obvious candidate where modem status data will be received, no really sure yet though.
– CH340/341 also have a USB INT endpoint but polling is also possible and, because some Android inner bugs, the best way to poll the lines state.

Let’s see how it works with an example

UsbDevice device;
UsbDeviceConnection usbConnection;
...
UsbSerialDevice serial = UsbSerialDevice.createUsbSerialDevice(device, usbConnection);
serial.open();
serial.setBaudRate(115200);
serial.setDataBits(UsbSerialInterface.DATA_BITS_8);
serial.setParity(UsbSerialInterface.PARITY_NONE);
serial.setFlowControl(UsbSerialInterface.FLOW_CONTROL_RTS_CTS);

This is basically the same as the previous versions but now the setFlowControl is meaningful. Now that We have our connection configured to pay attention to the RTS and CTS lines let’s define our callback to receive or status changes.

private UsbSerialInterface.UsbCTSCallback ctsCallback = new UsbSerialInterface.UsbCTSCallback() {
        @Override
        public void onCTSChanged(boolean state) {
            //Your code goes here!
        }
    };

And pass the reference to the UsbSerialDevice object

serial.getCTS(ctsCallback);

Now We know when the status of the line change. I will be also executed in the beginning to know what is the status of the line. If you need to raise the RTS or the DST lines jut write these lines.

serial.setRTS(true); // Raised
serial.setRTS(false); // Not Raised
serial.setDTR(true); // Raised
serial.setDTR(false); // Not Raised

PL2303, CH340/341 and CDC still lack of this feature. If you find something wrong just let me know. Happy crafting! 🙂