UART

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Serial data ports, called UARTs, transfer data between devices using two pins: TXD (transmit data) and RXD (receive data). UART stands for Universal Asynchronous Receiver Transmitter. Asynchronous means there is no clock signal to synchronize the two devices. The devices agree on a data rate, called the baud rate, and send a start bit the beginning of each transmitted character to keep the devices synchronized.

Tip

the TXD on one end (output) goes to the RXD on the other side (input) and vice versa.

The easiest way to test a UART is by wiring a device's TXD to its RXD so any transmitted data is received by the same device. This is called a "loopback" test. The following code performs a simple loopback test on the FEZ. If you connect the TX pin directly to the RX pin, you will see "ABCDEF" in the output window when running this code.

Tip

Don't forget to add the GHIElectronics.TinyCLR.Devices.Uart NuGet package!

using System.Diagnostics;
using System.Text;
using System.Threading;
using GHIElectronics.TinyCLR.Devices.Uart;
using GHIElectronics.TinyCLR.Pins;

class Program {
    private static void Main() {
        var txBuffer = new byte[] { 0x41, 0x42, 0x43, 0x44, 0x45, 0x46 };    //A, B, C, D, E, F
        var rxBuffer = new byte[txBuffer.Length];

        var myUart = UartController.FromName(FEZ.UartPort.Usart1);

        myUart.SetActiveSettings(9600, 8, UartParity.None, UartStopBitCount.One, UartHandshake.None);
        myUart.Enable();
        myUart.Write(txBuffer, 0, txBuffer.Length);

        while (true) {
            if (myUart.BytesToRead > 0) {
                var bytesReceived = myUart.Read(rxBuffer, 0, myUart.BytesToRead);
                Debug.WriteLine(Encoding.UTF8.GetString(rxBuffer, 0, bytesReceived));
            }
            Thread.Sleep(20);
        }
    }
}

Event Handlers

TinyCLR's UART API included the following event watchers:

• ClearToSendChanged
• DataReceived
• ErrorReceived

The following example demonstrates the DateReceived event.

using System.Diagnostics;
using System.Text;
using System.Threading;
using GHIElectronics.TinyCLR.Devices.Uart;
using GHIElectronics.TinyCLR.Pins;

class Program {
    private static byte[] txBuffer;
    private static byte[] rxBuffer;
    private static UartController myUart;

    private static void Main() {
        txBuffer = new byte[] { 0x41, 0x42, 0x43, 0x44, 0x45, 0x46 };    //A, B, C, D, E, F
        rxBuffer = new byte[txBuffer.Length];
        myUart = UartController.FromName(FEZ.UartPort.Usart1);

        myUart.SetActiveSettings(9600, 8, UartParity.None, UartStopBitCount.One, UartHandshake.None);
        myUart.Enable();

        myUart.DataReceived += MyUart_DataReceived;

        myUart.Write(txBuffer, 0, txBuffer.Length);

        while (true) {
            Thread.Sleep(20);
        }
    }

    private static void MyUart_DataReceived(UartController sender, DataReceivedEventArgs e) {
        var bytesReceived = myUart.Read(rxBuffer, 0, e.Count);
        Debug.WriteLine(Encoding.UTF8.GetString(rxBuffer, 0, bytesReceived));
    }
}

Tip

Once you type += after the event, hit the tab key and Visual Studio will automatically create the event for you.

RS232

UART uses the processor's voltage levels (logic levels) for transferring data. On the FEZ this is 0 to 3.3 volts. In the early days of computers UARTs used -12 to 12 volts to communicate reliably over longer distances. This is known as the RS232 standard.

Some PCs still include serial ports, but those are RS232 serial ports. A voltage level shifter is needed to properly connect a logic level UART to an RS232 device.

Warning

Connecting your device to an RS232 port without a proper voltage level shifter can damage your device.