Timer1 - A 16 bit timer used by the Servo library Timer2 - An 8 bit timer used by the Tone library The Mega boards have Timers 3,4,5 which may be used instead Calculations As these timers are hardware based, all timing is related to the clock of the timer. ESP32 Timer Interrupts. STM32 Timer Interrupts. The software example below will simply show the count of times it has fired, in the Serial Monitor, and is configured to fire once per second. The code in loop is simply to output to the user, and like with External Interrupts, loop can simply inspect the interrupts flag, and perform an action based on this as needed.
Therefore, higher rates of interrupts cause greater power consumption. Tickless OS. The so-called Tickless OS provides one solution to issue. The basic concept here is that the periodic, timer interrupt is eliminated and replaced with a one-shot, interval timer. It becomes event driven instead of polled: The default system timer is a polled design. On each interrupt, the NuttX logic checks if it needs to do anything and, if so, it does it.
Using an interval timer, one can anticipate when the next interesting OS event will occur, program the interval time and wait for it to fire. When the interval time fires, then the scheduled activity is performed. In order to use the Tickless OS, one must provide special support from the platform-specific code.
Just as with the default system timer, the platform-specific code must provide the timer resources to support the OS behavior. Currently these timer resources are only provided on a few platforms. These paragraphs will explain how to provide the Tickless OS support to any platform. The interval timer allows programming events to occur after an interval. With the alarm, you can set a time in the future and get an event when that alarm goes off. This option selects the use of an alarm. The advantage of an alarm is that it avoids some small timing errors; the advantage of the use of the interval timer is that the hardware requirement may be less.
However, the time is currently held in unsigned int. On some systems, this may be bits in width but on most contemporary systems it will be bits. So the trade-off is between range and resolution you could also modify the code to use a bit value if you really want both.
The tickless option can be supported either via a simple interval timer plus elapsed time or via an alarm. Since timers are a limited resource, the use of two timers could be an issue on some systems. The job could be done with a single timer if, for example, the single timer were kept in a free-running at all times. Then you could have both with a single timer: An alarm and a free-running counter with the same timer! In addition to these imported interfaces, the RTOS will export the following interfaces for use by the platform-specific interval timer implementation:.
Initializes all platform-specific timer facilities. The naming will depend on the architecture so for STM32 archname will be stm Assumptions : Called early in the initialization sequence before any special concurrency protections are required.
This function provides the basis for reporting the current time and also is used to eliminate error build-up from small errors in interval time calculations. Identifies the interrupt service routines to be invoked when the IRQ occurs. The field points to the first element of the list of irqaction descriptors associated with the IRQ. The irqaction descriptor is described later in the chapter.
A set of flags describing the IRQ line status see Table Shows 0 if the IRQ line is enabled and a positive value if it has been disabled at least once.
Counter of unhandled interrupt occurrences on the IRQ line for diagnostic use only. An interrupt is unexpected if it is not handled by the kernel, that is, either if there is no ISR associated with the IRQ line, or if no ISR associated with the line recognizes the interrupt as raised by its own hardware device.
Usually the kernel checks the number of unexpected interrupts received on an IRQ line, so as to disable the line in case a faulty hardware device keeps raising an interrupt over and over.
Because the IRQ line can be shared among several devices, the kernel does not disable the line as soon as it detects a single unhandled interrupt. The status of an IRQ line is described by the flags listed in Table An IRQ has occurred on the line; its occurrence has been acknowledged to the PIC, but it has not yet been serviced by the kernel. The kernel is using the IRQ line while performing a hardware device probe; moreover, the corresponding interrupt has not been raised.
This is accomplished through the following statements:. This code looks in the interrupt array to find the interrupt handler addresses that it uses to set up the interrupt gates. The advantage of this object-oriented approach is that drivers need not to be aware of the kind of PIC installed in the system.
Each driver-visible interrupt source is transparently wired to the appropriate controller. This variable is initialized as follows:. Next come the pointers to six different functions used to program the PIC. The first two functions start up and shut down an IRQ line of the chip, respectively. But in the case of the A chip, these functions coincide with the third and fourth functions, which enable and disable the line. As described earlier, multiple devices can share a single IRQ.
Therefore, the kernel maintains irqaction descriptors see Figure earlier in this chapter , each of which refers to a specific hardware device and a specific interrupt. The fields included in such descriptor are shown in Table , and the flags are shown in Table This is the key field that allows many devices to share the same IRQ.
Points to the next element of a list of irqaction descriptors. The elements in the list refer to hardware devices that share the same IRQ. The device may be considered a source of events that occurs randomly; it can thus be used by the kernel random number generator. Set of flags denoting the pending softirqs see the section " Softirqs " later in this chapter. Number of occurrences of NMI interrupts.
Linux sticks to the Symmetric Multiprocessing model SMP ; this means, essentially, that the kernel should not have any bias toward one CPU with respect to the others. As a consequence, the kernel tries to distribute the IRQ signals coming from the hardware devices in a round-robin fashion among all the CPUs.
In particular, the task priority register TPR of each chip is initialized to a fixed value, meaning that the CPU is willing to handle every kind of IRQ signal, regardless of its priority. The Linux kernel never modifies this value after its initialization. All task priority registers contain the same value, thus all CPUs always have the same priority.
Because such values are automatically changed after every interrupt, the IRQ signals are, in most cases, fairly distributed among all CPUs. No other CPUs are notified of the event. All this is magically done by the hardware, so it should be of no concern for the kernel after multi-APIC system initialization.
Unfortunately, in some cases the hardware fails to distribute the interrupts among the microprocessors in a fair way for instance, some Pentium 4-based SMP motherboards have this problem. Therefore, Linux 2. The exception stack is used when handling exceptions including system calls. The hard IRQ stack is used when handling interrupts. As with other context switches, the need to save registers leaves the kernel developer with a somewhat messy coding job, because the registers have to be saved and restored using assembly language code.
However, within those operations, the processor is expected to call and return from a C function. Some interrupt handler software might reset the machine, or just display the error, but most interrupt handlers do one of 3 things: The hardware such as a video display signals it is ready for the next piece of data, the interrupt handler sends out the next piece of data, then returns to the process that was interrupted.
The hardware such as a keyboard signals that it has a piece of data available, the interrupt handler reads in that piece of data, then returns to the process that was interrupted. The timer interrupt handler runs the OS scheduler. If the process that was just interrupted has used up its time quantum, and there is some other runnable process, then the scheduler "returns" to that other process. Later, when the timer interrupts some other process, the scheduler will "return" to this process.
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