timers.txt

一款MP3 Player Firmware 的原代码,非常有参考价值

There are now timers available for use.  There are four timers available,
each of which counts down from a user-defined value to zero at a rate of
10 counts/second.  Each timer is 16 bits, allowing timouts of up to 1
hour and 49 minutes.  When a timer reaches zero, it will generate an
event and remain at zero until set again.

These three functions access the four timers:

    void set_timer(unsigned char timer, unsigned int value);

Set a timer to a particular value and start it counting down.

    unsigned int read_timer(unsigned char timer);

Read the current 16 bit count for a timer.

    void clear_timer(unsigned char timer);

Stop a timer and clear its count to zero.  No event is generated.


The new timer code also keeps a 32 bit system clock, which
increments at 10 Hz (will overflow in 13.6 years).  This may 
be read with the following function and readings may be
subtracted from each other for long duration time measurement.

    unsigned long clock_tick(void);