Limit occurrences of an event - c++

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
}
}
}
}

Answer 2

You could keep a static variable in that function storing the last time the switch was triggered.
Then all you need to do is check that the current time is at least x milliseconds after that time.
You could use GetSystemTimeAsFileTime or GetSystemTime to get the current timestamp, which is supposed to have millisecond resolution.

Answer 3

If you could store the millisecond interval between flashes in a global variable, say FLASH_INETRVAL:
void WINAPI flash (HINSTANCE hThisInstance,
HINSTANCE hPrevInstance,
LPSTR lpszArgument,
int nFunsterStil)
{
HANDLE hSerial;
static long lastFlashMillis;
// currentTimeMillis() should be defined using a system
// call that returns current
// system time in milliseconds.
long interval = currentTimeMillis() - lastFlashMillis;
if (interval < FLASH_INTERVAL)
Sleep (interval);
lastFlashMillis = currentTimeMillis();
//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);
}