NUC029LAN Datasheet, PDF (35/99 Pages) List of Unclassifed Manufacturers – 32-bit Microcontroller

NUC029
5.2.6 System Timer (SysTick)
The Cortex^® -M0 includes an integrated system timer, SysTick, which provides a simple, 24-bit
clear-on-write, decrementing, wrap-on-zero counter with a flexible control mechanism. The
counter can be used as a Real Time Operating System (RTOS) tick timer or as a simple counter.
When system timer is enabled, it will count down from the value in the SysTick Current Value
Register (SYST_CVR) to 0, and reload (wrap) to the value in the SysTick Reload Value Register
(SYST_RVR) on the next clock cycle, then decrement on subsequent clocks. When the counter
transitions to 0, the COUNTFLAG status bit is set. The COUNTFLAG bit clears on reads.
The SYST_CVR value is UNKNOWN on reset. Software should write to the register to clear it to 0
before enabling the feature. This ensures the timer will count from the SYST_RVR value rather
than an arbitrary value when it is enabled.
If the SYST_RVR is 0, the timer will be maintained with a current value of 0 after it is reloaded
with this value. This mechanism can be used to disable the feature independently from the timer
enable bit.
For more detailed information, please refer to the “ARM^® Cortex^® -M0 Technical Reference
Manual” and “ARM^® v6-M Architecture Reference Manual”.
5.2.7 Nested Vectored Interrupt Controller (NVIC)
The Cortex^® -M0 provides an interrupt controller as an integral part of the exception mode, named
as “Nested Vectored Interrupt Controller (NVIC)”, which is closely coupled to the processor core
and provides following features:
 Nested and Vectored interrupt support
 Automatic processor state saving and restoration
 Reduced and deterministic interrupt latency
The NVIC prioritizes and handles all supported exceptions. All exceptions are handled in “Handler
Mode”. This NVIC architecture supports 32 (IRQ[31:0]) discrete interrupts with 4 levels of priority.
All of the interrupts and most of the system exceptions can be configured to different priority
levels. When an interrupt occurs, the NVIC will compare the priority of the new interrupt to the
current running one’s priority. If the priority of the new interrupt is higher than the current one, the
new interrupt handler will override the current handler.
When an interrupt is accepted, the starting address of the interrupt service routine (ISR) is fetched
from a vector table in memory. There is no need to determine which interrupt is accepted and
branch to the starting address of the correlated ISR by software. While the starting address is
fetched, NVIC will also automatically save processor state including the registers “PC, PSR, LR,
R0~R3, R12” to the stack. At the end of the ISR, the NVIC will restore the mentioned registers
from stack and resume the normal execution. Thus it will take less and deterministic time to
process the interrupt request.
The NVIC supports “Tail Chaining” which handles back-to-back interrupts efficiently without the
overhead of states saving and restoration and therefore reduces delay time in switching to
pending ISR at the end of current ISR. The NVIC also supports “Late Arrival” which improves the
efficiency of concurrent ISRs. When a higher priority interrupt request occurs before the current
ISR starts to execute (at the stage of state saving and starting address fetching), the NVIC will
give priority to the higher one without delay penalty. Thus it advances the real-time capability.
For more detailed information, please refer to the “ARM^® Cortex^® -M0 Technical Reference
Manual” and “ARM^® v6-M Architecture Reference Manual”.
May 18, 2015
Page 35 of 99
Rev 1.02