Android Things Raspberry Pi UART 可靠性问题

Question

我正在通过 UART 从 Arduino 接收数据。我按照文档进行了操作，并且大部分时间都按预期获得了数据。有时读取没有完成，得到几个零，然后用其余数据开始新的读取。这可以在示例输出中看到，所有数据都在那里，但分成 2 次读取。我每秒只发送一次数据，所以应该有足够的时间。

我的代码：

private UartDeviceCallback mUartCallback = new UartDeviceCallback() {
    @Override
    public boolean onUartDeviceDataAvailable(UartDevice uart) {
        // Read available data from the UART device
        try {
            readUartBuffer(uart);
        } catch (IOException e) {
            Log.w(TAG, "Unable to access UART device", e);
        }
        // Continue listening for more interrupts
        return true;
    }

    private void readUartBuffer(UartDevice uart) throws IOException {
        // Maximum amount of data to read at one time
        final int maxCount = 20;
        byte[] buffer = new byte[maxCount];

        uart.read(buffer, maxCount);
        Log.i(TAG, Arrays.toString(buffer));
        }

    @Override
    public void onUartDeviceError(UartDevice uart, int error) {
        Log.w(TAG, uart + ": Error event " + error);
    }
};

示例输出：

    [50, 48, 54, 46, 52, 53, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 54, 46, 57, 51, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 54, 46, 48, 52, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 55, 46, 51, 52, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 54, 46, 53, 48, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 55, 46, 51, 54, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]
    [50, 48, 54, 46, 57, 51, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 0, 0, 0, 0]
    [51, 48, 32, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    [50, 48, 55, 46, 51, 56, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 0, 0, 0, 0]
    [51, 48, 32, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    [50, 48, 54, 46, 52, 57, 32, 50, 49, 46, 55, 48, 32, 51, 51, 46, 51, 48, 32, 0]

我很确定问题出在 R Pi，因为我从 Arduino 循环回到我的 PC 没有任何问题。我还发现，除非我将 maxCount 设为我发送的确切字节数，否则问题会更加普遍。因此，数据以随机包的形式出现，但顺序正确。我在浪费时间吗？我应该只使用 I2C 吗？

Answer 1

“我应该只使用 I2C 吗？” - 不。 R Pi 没有问题，因为“所有数据都在那里”。它们（可以）拆分（或不拆分，特别是如果它很短）为 2 个（或更多）读取，因为 onUartDeviceDataAvailable() 可以在所有数据可用（但只有一部分可用）之前被触发，所以你应该阅读它们一个循环，直到您收到所有这些。而且，从您的代码中：maxCount - Maximum amount of data to read at one time 不是所有数据的大小，它是最大值。一次性读取的大小。您的代码可以是这样的（注意！这只是示例，不是完整的解决方案）：

private void readUartBuffer(UartDevice uart) throws IOException {
    // Buffer for all data
    final int maxSizeOfAllData = 30;
    byte[] completaDataBuffer = new byte[maxSizeOfAllData];

    // Buffer for one uart.read() call
    final int maxCount = 20;
    byte[] buffer = new byte[maxCount];

    int bytesReadOnce;     // number of actually available data
    int totalBytesRead = 0;

    // read all available data  
    while ((bytesReadOnce = uart.read(buffer, maxCount))) > 0) {
        // add new  data to "all data" buffer
        for (int i = 0; i < bytesReadOnce; i++) {
            completaDataBuffer[totalBytesRead + i] = buffer[i] 

            if (totalBytesRead + i == maxSizeOfAllData - 1) {
                // process complete buffer here  
                ...
                totalBytesRead = 0;
                break;
            }
        }
        totalBytesRead += bytesReadOnce;
    }
}

另外，看看Android Things user-space drivers 中的NmeaGpsModule.java 和Android Things samples 中的LoopbackActivity.java。

Answer 2

我最终添加了一个结束字符 (0x36) 并使用了 dataCompleteFlag：

       private void readUartBuffer(UartDevice uart) throws IOException {
        // Maximum amount of data to read at one time
        final int maxCount = 32;
        byte[] buffer = new byte[maxCount];
        boolean dataCompleteFlag = false;

        uart.read(buffer, maxCount);
        Log.i(TAG, Arrays.toString(buffer));

        if (!dataCompleteFlag) {
            for (int i = 0; i < maxCount; i++) {
                if (buffer[i] == 36) {
                    dataCompleteFlag = true;
                    dataCount = 0;
                }
                else if(dataCount > maxCount) {
                    dataCount = 0;
                }
                else if(buffer[i] != 0) {
                    finalDataBuffer[dataCount] = buffer[i];
                    dataCount++;
                }
            }
        }

        if (dataCompleteFlag) {
            //process data
        }
    }

    @Override
    public void onUartDeviceError(UartDevice uart, int error) {
        Log.w(TAG, uart + ": Error event " + error);
    }
};