Timeslice and task scheduler timing – Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement User Manual
Page 220
High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
TIMESLICE AND TASK SCHEDULER TIMING
The task scheduler is primarily driven by the timer 0. The exported ROM function
tick_init handles the initial timer 0 configuration. The
tick_init routine configures timer 0 as a 16-bit timer running from an (oscillator frequency / 12) input clock, with an initial start value of
TH0:TL0 = FA00h. Timer 0 is then enabled as a high-priority interrupt source and started (TR0 = 1). Once running, the timer 0 interrupt
is triggered at a periodic interval so that it can be determined whether the scheduler needs to be run and whether it should be run
given the current CPU activity. This periodic interval is equal to (65,536d – FA00h)
✕
1 / (crystal frequency)
✕
12 and is defined as the
basic timeslice. The exported ROM function
WOS_tick is the default timer 0 interrupt service routine that is executed each timeslice.
The
WOS_tick routine updates its millisecond counter and signals that the task scheduler should be run, once the fourth timeslice has
elapsed. At this point, the task scheduler is run unless other interrupts are in progress, in which case the task scheduler is deferred. If
the scheduler is deferred, its ability to run is reassessed on each of the following (timer 0 interrupt) timeslices or whenever a task is
suspended or put to sleep. Note that the
tick_init function is automatically called by the rom_init function. Figure 23-1 illustrates the
general flow that occurs on each (timer 0 interrupt) timeslice.
Figure 23-1. Timer 0 Interrupt Routine (WOS_tick) Flow
220
Stop TIMER 0
Software reload of TH0:TL0
Restart TIMER 0
Increment millisecond counter (ms_count_4:0)
Already know that task scheduler is required?
(need sched = 1) from previous deferral?
Millisecond counter = 4?
Low-priority interrupt
currently in progress?
--OR
Critical task currently in
progress? (wos_crit_count > 0)
Set bit need_sched = 1.
TASK SCHEDULER
Return to application
Y
N
N
Y
Y
N
Maxim Integrated