直入主题 看看你的需求是这样的吗

String为Arduino的String 

String string = "020104C0";

变成下面这个

byte[4] bytes = {0x02,0x01,0x04,0xC0};

是的话，自己Copy。 用法在最后。

void hexCharacterStringToBytes(byte *byteArray, const char *hexString)
{
    bool oddLength = strlen(hexString) & 1;

    byte currentByte = 0;
    byte byteIndex = 0;

    for (byte charIndex = 0; charIndex < strlen(hexString); charIndex++)
    {
        bool oddCharIndex = charIndex & 1;

        if (oddLength)
        {
            // If the length is odd
            if (oddCharIndex)
            {
                // odd characters go in high nibble
                currentByte = nibble(hexString[charIndex]) << 4;
            }
            else
            {
                // Even characters go into low nibble
                currentByte |= nibble(hexString[charIndex]);
                byteArray[byteIndex++] = currentByte;
                currentByte = 0;
            }
        }
        else
        {
            // If the length is even
            if (!oddCharIndex)
            {
                // Odd characters go into the high nibble
                currentByte = nibble(hexString[charIndex]) << 4;
            }
            else
            {
                // Odd characters go into low nibble
                currentByte |= nibble(hexString[charIndex]);
                byteArray[byteIndex++] = currentByte;
                currentByte = 0;
            }
        }
    }
}

byte nibble(char c)
{
    if (c >= '0' && c <= '9')
        return c - '0';

    if (c >= 'a' && c <= 'f')
        return c - 'a' + 10;

    if (c >= 'A' && c <= 'F')
        return c - 'A' + 10;

    return 0; // Not a valid hexadecimal character
}

怕你不知道，展示下用法。

String data = "020104C0";
//这里长度记得除下2    使用前校验一下String的长度是否正确
byte byteArray[data.length() / 2] = {0};
hexCharacterStringToBytes(byteArray, data.c_str());

两个参数，第一个接收byte的数组,第二个要String.c_str()

好了 完事，C语言是世界上最摆烂的语言！！！