timer_start()

Description:

Read and return the hardware timer's current count value.

Timer functions in startup code

Examples

The following code measures how long it takes to perform an action and converts the value to nanoseconds:

unsigned start = timer_start(); 
// Perform the action. 
unsigned num_hardware_timer_ticks = timer_diff(start);
unsigned long elapsed_time_in_ns = timer_tick2ns(num_hardware_timer_ticks);

The following code adds a waiting period measured in nanoseconds. To compare the specified waiting period to the value returned by timer_diff(), it converts the nanoseconds to hardware timer ticks:

unsigned timer_periods = timer_ns2tick(time_to_wait_in_ns);
const unsigned start = timer_start();
do {
	// nothing
} while(timer_diff(start) < timer_periods);

Timer reset

When you implement timer_start(), you can choose whether or not it resets the hardware timer each time you call it. For this reason, you cannot nest calls to timer_start() and timer_diff(). For example, the following code produces unexpected results:

// DO NOT DO THIS
unsigned start_overall = timer_start(); 			// Reset hardware timer.
for(unsigned i =0; i < NUM_THINGS; i++) { 
	unsigned start = timer_start(); 				// Reset hardware timer again. 
	// do stuff 
	unsigned diff = timer_diff(start); 
	// do more stuff
}
// do other stuff
unsigned diff_overall = timer_diff(start_overall); // Invalid value.

Interrupts

Because interrupts are not enabled until the kernel is running, they are disabled when these startup functions execute.

Returns:

A value in hardware timer ticks to pass to timer_diff().