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author | Gregory Nutt <gnutt@nuttx.org> | 2014-08-10 11:34:20 -0600 |
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committer | Gregory Nutt <gnutt@nuttx.org> | 2014-08-10 11:34:20 -0600 |
commit | 1869a50dc6bfe5ff97b0f66d7458fabd6d04fbe8 (patch) | |
tree | 5db763f716b54acef7a32d68ab6ac89ef76c9d96 /nuttx/configs/sama5d4-ek/README.txt | |
parent | b399e981ac2ab1154dc59021eb942781868ddae0 (diff) | |
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Update README files
Diffstat (limited to 'nuttx/configs/sama5d4-ek/README.txt')
-rw-r--r-- | nuttx/configs/sama5d4-ek/README.txt | 57 |
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diff --git a/nuttx/configs/sama5d4-ek/README.txt b/nuttx/configs/sama5d4-ek/README.txt index 26c6c1154..7d0cb70ba 100644 --- a/nuttx/configs/sama5d4-ek/README.txt +++ b/nuttx/configs/sama5d4-ek/README.txt @@ -94,6 +94,7 @@ Contents - TRNG and /dev/random - Audio Support - TM7000 LCD/Touchscreen + - Tickless OS - SAMA4D4-EK Configuration Options - Configurations - To-Do List @@ -3108,6 +3109,62 @@ TM7000 LCD/Touchscreen --- To be provided. +Tickless OS +=========== + + By default, a NuttX configuration uses a periodic timer interrupt that + drives all system timing. The timer is provided by architecture-specifi + code that calls into NuttX at a rate controlled by CONFIG_USEC_PER_TICK. + The default value of CONFIG_USEC_PER_TICK is 10000 microseconds which + corresponds to a timer interrupt rate of 100 Hz. + + An option is to configure NuttX to operation in a "tickless" mode. Some + limitations of default system timer are, in increasing order of + importance: + + - Overhead: Although the CPU usage of the system timer interrupt at 100Hz + is really very low, it is still mostly wasted processing time. One most + timer interrupts, there is really nothing that needs be done other than + incrementing the counter. + - Resolution: Resolution of all system timing is also determined by + CONFIG_USEC_PER_TICK. So nothing that be time with resolution finer than + 10 milliseconds be default. To increase this resolution, + CONFIG_USEC_PER_TICK an be reduced. However, then the system timer + interrupts use more of the CPU bandwidth processing useless interrupts. + - Power Usage: But the biggest issue is power usage. When the system is + IDLE, it enters a light, low-power mode (for ARMs, this mode is entered + with the wfi or wfe instructions for example). But each interrupt + awakens the system from this low power mode. Therefore, higher rates + of interrupts cause greater power consumption. + + 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. + + The following configuration options will enable support for the Tickless + OS for the SAMA5D platforms using TC0 channels 0-3 (other timers or + timer channels could be used making the obvious substitutions): + + RTOS Features -> Clocks and Timers + CONFIG_SCHED_TICKLESS=y : Configures the RTOS in tickless mode + + System Type -> SAMA5 Peripheral Support + CONFIG_SAMA5_TC0=y : Enable TC0 (TC channels 0-3 + + System Type -> Timer/counter Configuration + CONFIG_SAMA5_ONESHOT=y : Enables one-shot timer wrapper + CONFIG_SAMA5_FREERUN=y : Enabled free-running timer wrapper + CONFIG_SAMA5_TICKLESS_ONESHOT=0 : Selects TC0 channel 0 for the one-shot + CONFIG_SAMA5_TICKLESS_FREERUN=1 : Selects TC0 channel 1 for the free- + : running timer + SAMA4D4-EK Configuration Options ================================= |