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Timer Driver (PIT)

The Programmable Interval Timer (PIT) is used for timekeeping and preemption.

1. Hardware Details

  • Frequency: 1.193182 MHz.
  • IRQ: 0 (Interrupt Vector 32).
  • Ports: 0x40 (Channel 0), 0x43 (Mode/Command Register).

2. Implementation

  • timer_init(uint32_t frequency): Configures the PIT to fire interrupts at a specific rate (e.g., 1000Hz = every 1ms).
  • onIrq0(InteruptReg *regs): The interrupt handler called on every tick.
  • Increments a global ticks counter.
  • Iterates through the active Triggers.

3. Trigger System

TilekarOS includes a "Trigger" system to schedule periodic tasks without complex multitasking overhead.

Creating a Trigger

insert_triger(tick_mod, func, flags): Registers a function func to be called every tick_mod ticks.

  • Flags:
  • TIMER_TRIGGER_ACTIVE: The trigger is currently running.
  • TIMER_TRIGGER_USE_GLOBAL: Use the absolute system ticks instead of a local counter.

Example

The scheduler uses a trigger to preempt tasks every 100ms: 

scheduler_trigger_index = insert_triger(100, &task_yield, 0);

4. Timekeeping

The timer provides basic timekeeping through:

  • get_ticks(): Returns the number of milliseconds since boot.
  • set_ticks(uint32_t): Manually updates the tick counter.