967acc9b20
Prior to this commit, if something other than systick wakes the CPU from tickless idle, vPortSuppressTicksAndSleep() might cause xTickCount to increment once too many times. See "bug 2" in this forum post: https://forums.freertos.org/t/ultasknotifytake-timeout-accuracy/9629/40 SysTick ------- The SysTick is the hardware timer that provides the OS tick interrupt in the official ports for Cortex M. SysTick starts counting down from the value stored in its reload register. When SysTick reaches zero, it requests an interrupt. On the next SysTick clock cycle, it loads the counter again from the reload register. To get periodic interrupts every N SysTick clock cycles, the reload register must be N - 1. Bug Example ----------- - CPU is sleeping in vPortSuppressTicksAndSleep() - Something other than the SysTick wakes the CPU. - vPortSuppressTicksAndSleep() calculates the number of SysTick counts until the next tick. The bug occurs only if this number is small. - vPortSuppressTicksAndSleep() puts this small number into the SysTick reload register, and starts SysTick. - vPortSuppressTicksAndSleep() calls vTaskStepTick() - While vTaskStepTick() executes, the SysTick expires. The ISR pends because interrupts are masked, and SysTick starts a 2nd period still based on the small number of counts in its reload register. This 2nd period is undesirable and is likely to cause the error noted below. - vPortSuppressTicksAndSleep() puts the normal tick duration into the SysTick's reload register. - vPortSuppressTicksAndSleep() unmasks interrupts before the SysTick starts a new period based on the new value in the reload register. [This is a race condition that can go either way, but for the bug to occur, the race must play out this way.] - The pending SysTick ISR executes and increments xPendedTicks. - The SysTick expires again, finishing the second very small period, and starts a new period this time based on the full tick duration. - The SysTick ISR increments xPendedTicks (or xTickCount) even though only a tiny fraction of a tick period has elapsed since the previous tick. The bug occurs when *two* consecutive small periods of the SysTick are both counted as ticks. The root cause is a race caused by the small SysTick period. If vPortSuppressTicksAndSleep() unmasks interrupts *after* the small period expires but *before* the SysTick starts a period based on the full tick period, then two small periods are counted as ticks when only one should be counted. The end result is xTickCount advancing nearly one full tick more than time actually elapsed as measured by the SysTick. This is not the kind of time slippage normally associated with tickless idle. After this commit the code starts the SysTick and then immediately modifies the reload register to ensure the very short cycle (if any) is conducted only once. This strategy requires special consideration for the build option that configures SysTick to use a divided clock. To avoid waiting around for the SysTick to load value from the reload register, the new code temporarily configures the SysTick to use the undivided clock. The resulting timing error is typical for tickless idle. The error (commonly known as drift or slippage in kernel time) caused by this strategy is equivalent to one or two counts in ulStoppedTimerCompensation. This commit also updates comments and #define symbols related to the SysTick clock option. The SysTick can optionally be clocked by a divided version of the CPU clock (commonly divide-by-8). The new code in this commit adjusts these comments and symbols to make them clearer and more useful in configurations that use the divided clock. The fix made in this commit requires the use of these symbols, as noted in the code comments. |
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|---|---|---|
| .github | ||
| include | ||
| portable | ||
| CONTRIBUTING.md | ||
| croutine.c | ||
| event_groups.c | ||
| GitHub-FreeRTOS-Kernel-Home.url | ||
| History.txt | ||
| LICENSE.md | ||
| list.c | ||
| queue.c | ||
| Quick_Start_Guide.url | ||
| README.md | ||
| SECURITY.md | ||
| stream_buffer.c | ||
| tasks.c | ||
| timers.c | ||
Getting started
This repository contains FreeRTOS kernel source/header files and kernel ports only. This repository is referenced as a submodule in FreeRTOS/FreeRTOS repository, which contains pre-configured demo application projects under FreeRTOS/Demo directory.
The easiest way to use FreeRTOS is to start with one of the pre-configured demo application projects. That way you will have the correct FreeRTOS source files included, and the correct include paths configured. Once a demo application is building and executing you can remove the demo application files, and start to add in your own application source files. See the FreeRTOS Kernel Quick Start Guide for detailed instructions and other useful links.
Additionally, for FreeRTOS kernel feature information refer to the Developer Documentation, and API Reference.
Getting help
If you have any questions or need assistance troubleshooting your FreeRTOS project, we have an active community that can help on the FreeRTOS Community Support Forum.
Cloning this repository
To clone using HTTPS:
git clone https://github.com/FreeRTOS/FreeRTOS-Kernel.git
Using SSH:
git clone git@github.com:FreeRTOS/FreeRTOS-Kernel.git
Repository structure
-
The root of this repository contains the three files that are common to every port - list.c, queue.c and tasks.c. The kernel is contained within these three files. croutine.c implements the optional co-routine functionality - which is normally only used on very memory limited systems.
-
The
./portabledirectory contains the files that are specific to a particular microcontroller and/or compiler. See the readme file in the./portabledirectory for more information. -
The
./includedirectory contains the real time kernel header files.