| 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 After this operation is complete, the kernel scheduler is no longer in effect.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 | |
