OS21, ARM

Timers

8083358

5/14

2

Timers

2.1

Timers overview

In order to run, OS21 requires at least three independent timers. Each of these must be

capable of running as a free running, auto-reload counter, with interrupt on underflow. Each

is programmed to count some fraction of the input clock.

The greatest accuracy is obtained by counting based on a large fraction of the input clock,

and running that clock at high frequency.

2.2

Input clock frequency

The precise speed of the input clock is determined by the end user; it is a function of the

board design and boot software. OS21 is not responsible for setting the input speed,

therefore it has to be made aware of what it is.

This is done with the Board Support Package (BSP) using a function called

bsp_timer_input_clock_frequency_hz

(). Full details on how to use this function can

be found in the OS21 User manual (ADCS 7358306), chapter 16.

2.3

OS21 tick duration

OS21 establishes the period of one tick when it boots. Based on the input clock frequency it

selects an appropriate divisor to yield a tick which is approximately 10 microseconds.

2.4

ARM timer assignments

How OS21 uses the ARM timers depends upon the exact hardware available. Table 1 shows

an example of how the timers (four in this case) are assigned in a typical configuration.

The system timer is left free running and is used by time_now() to return the system time.

On ARM platforms, the system time (osclock_t) is a 64-bit value. OS21 maintains the top

32 bits of the 64-bit time via an interrupt handler which is called each time the 32-bit timer

reaches zero. The lower 32 bits of the system time are the value in the system timer.

The timeslice timer is programmed to run for the timeslice period before generating an

interrupt and reloading. This is used to drive timeslice events into the task scheduler.

Table 1.

ARM timer assignments

Timer name

OS21 usage

Timer0

System timer

Timer1

Timeslice timer

Timer2

Timeout timer

Timer3

OS21 Profiling timer