Книга: Real-Time Concepts for Embedded Systems

11.7 Soft Timers and Timer Related Operations

11.7 Soft Timers and Timer Related Operations

Many RTOSs provide a set of timer-related operations for external software components and applications through API sets. These common operations can be cataloged into these groups:

· group 1 - provides low-level hardware related operations,

· group 2 - provides soft-timer-related services, and

· group 3 - provides access either to the storage of the real-time clock or to the system clock.

Not all of the operations in each of these three groups, however, are offered by all RTOSs, and some RTOSs provides additional operations not mentioned here.

The first group of operations is developed and provided by the BSP developers. The group is considered low-level system operations. Each operation in the group is given a fictitious function name for this discussion. Actual function names are implementation dependent.

Table 11.1: Group 1 Operations.

Typical Operations Description
sys_timer_enable Enables the system timer chip interrupts. As soon as this operation is invoked, the timer interrupts occur at the preprogrammed frequency, assuming that the timer chip has been properly initialized with the desired values. Only after this operation is complete can kernel task scheduling take place.
sys_timer_disable Disables the system timer chip interrupts. After this operation is complete, the kernel scheduler is no longer in effect. Other system-offered services based on time ticks are disabled by this operation as well.
sys_timer_connect Installs the system timer interrupt service routine into the system exception vector table. The new timer ISR is invoked automatically on the next timer interrupt. The installed function is either part of the BSP or the kernel code and represents the 'timer ISR' depicted in Figure 11.3, page 172 .
Input Parameters: 1. New timer interrupt service routine
sys_timer_getrate Returns the system clock rate as the number of ticks per second that the timer chip is programmed to generate.
Output Parameter: 1. Ticks per second
sys_timer_setrate Sets the system clock rate as the number of ticks per second the timer chip generates. Internally, this operation reprograms the PIT to obtain the desired frequency.
Input Parameters: 1. Ticks per second
sys_timer_getticks Returns the elapsed timer ticks since system power up. This figure is the total number of elapsed timer ticks since the system was first powered on.
Output Parameters:1. Total number of elapsed timer ticks

The second group of timer-related operations includes the core timer operations that are heavily used by both the system modules and applications. Either an independent timer-handling facility or a built-in one that is part of the kernel offers these operations. Each operation in the group is given a fictitious function name for this discussion. Actual function names are implementation dependent.

The timer_create and timer_start operations allow the caller to start a timer that expires some time in the future. The caller-supplied function is invoked at the time of expiration, which is specified as a time relative with respect to when the timer_start operation is invoked. Through these timer operations, applications can install soft timers for various purposes. For example, the TCP protocol layer can install retransmission timers, the IP protocol layer can install packet-reassembly discard timers, and a device driver can poll an I/O device for input at predefined intervals.

Table 11.2: Group 2 Operations.

Typical Operations Description
timer_create Creates a timer. This operation allocates a soft-timer structure. Any software module intending to install a soft timer must first create a timer structure. The timer structure contains control information that allows the timer-handling facility to update and expire soft timers. A timer created by this operation refers to an entry in the soft-timers array depicted in Figure 11.3.
Input Parameter: Expiration time. User function to be called at the timer expiration
Output Parameter: An ID identifying the newly created timer structure
Note: This timer structure is implementation dependent. The returned timer ID is also implementation dependent.
timer_delete Deletes a timer. This operation deletes a previously created soft timer, freeing the memory occupied by the timer structure.
Input Parameter: 1. An ID identifying a previously created timer structure
Note: This timer ID is implementation dependent.
timer_start Starts a timer. This operation installs a previously created soft timer into the timer-handling facility. The timer begins running at the completion of this operation.
Input Parameter: 1. An ID identifying a previously created timer structure
timer_cancel Cancels a currently running timer. This operation cancels a timer by removing the currently running timer from the timer-handling facility.
Input Parameter: 1. An ID identifying a previously created timer structure 

The third group is mainly used by user-level applications. The operations in this group interact either with the system clock or with the real-time clock. A system utility library offers these operations. Each operation in the group is given a fictitious function name for this discussion. Actual function names are implementation dependent.

Table 11.3: Group 3 Operations.

Typical Operations Description
clock_get_time Gets the current clock time, which is the current running value either from the system clock or from the real-time clock.
Output Parameter: A time structure containing seconds, minutes, or hours[3]
clock_set_time Sets the clock to a specified time. The new time is set either into the system clock or into the real-time clock.
Input Parameter: A time structure containing seconds, minutes, or hours[3]

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