Question

I have a function that controls a bulb. The bulb is programmed to flash whenever a key is pressed. However, I want to limit the minimum interval between flashes to prevent the bulb from burning out. The bulb is controlled by a relay switch connected to the serial port, and the code is as follows:

void WINAPI flash (HINSTANCE hThisInstance, HINSTANCE hPrevInstance, LPSTR lpszArgument, int nFunsterStil)
{
    //MATT: Define the serial port procedure
    HANDLE hSerial;

    //MATT: Fire the flash (by initialising and uninitialising the port)
    hSerial = CreateFile("COM1",GENERIC_WRITE, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0); CloseHandle(hSerial);
}

How do I limit the minimum flash interval in milliseconds (millisecond accuracy is very important)?

Answer 1

You could use a simple variable that keeps the time as reported by QueryPerformanceCounter. The accuracy of QPC is very, very high on most systems. On my system, the frequency is 2.8million- or one tick per ten processor clocks.

class bulb {
    __int64 clocks;
    __int64 frequency;
public:
    static const int max_ms_between_flashes = 1;
    bulb() {
        LARGE_INTEGER li;
        QueryPerformanceFrequency(&li);
        frequency = li.QuadPart;
        clocks = 0;
    }
    void flash(...) {
        LARGE_INTEGER li;
        QueryPerformanceCounter(&li);
        if (clocks == 0) { 
            // We are the first time, so set the clocks var
            // and flash the bulb
            clocks = li.QuadPart;       
        } else {
            __int64 timepassed = clocks - li.QuadPart;
            if (timepassed >= (((double)frequency) / (max_ms_between_flashes * 1000))) {
                // It's been more than 1ms
                clocks = li.QuadPart;
                // flash the bulb
            }
        }
    }
}