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Functional Description
123
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Cortex-M4 Peripherals
part of a dynamic clock management control loop.
The timer consists of three registers:
•
SysTick Control and Status (STCTRL) : A control and status counter to configure its clock, enable the
counter, enable the SysTick interrupt, and determine counter status.
•
SysTick Reload Value (STRELOAD) : The reload value for the counter, used to provide the counter's
wrap value.
•
SysTick Current Value (STCURRENT) : The current value of the counter.
When enabled, the timer counts down on each clock from the reload value to zero, reloads (wraps) to the
value in the STRELOAD register on the next clock edge, then decrements on subsequent clocks. Clearing
the STRELOAD register disables the counter on the next wrap. When the counter reaches zero, the
COUNT status bit is set. The COUNT bit clears on reads.
Writing to the STCURRENT register clears the register and the COUNT status bit. The write does not
trigger the SysTick exception logic. On a read, the current value is the value of the register at the time the
register is accessed.
The SysTick counter runs on the system clock. If this clock signal is stopped for low power mode, the
SysTick counter stops. Ensure software uses aligned word accesses to access the SysTick registers.
The SysTick counter reload and current value are undefined at reset; the correct initialization sequence for
the SysTick counter is:
1. Program the value in the STRELOAD register.
2. Clear the STCURRENT register by writing to it with any value.
3. Configure the STCTRL register for the required operation.
NOTE:
When the processor is halted for debugging, the counter does not decrement.
2.2.2 Nested Vectored Interrupt Controller (NVIC)
This section describes the Nested Vectored Interrupt Controller (NVIC) and the registers it uses. The
NVIC supports:
•
109 interrupts
•
A programmable priority level of 0 to 7 for each interrupt. A higher level corresponds to a lower priority,
so level 0 is the highest interrupt priority.
•
Low-latency exception and interrupt handling
•
Level and pulse detection of interrupt signals
•
Dynamic reprioritization of interrupts
•
Grouping of priority values into group priority and subpriority fields
•
Interrupt tail-chaining
•
An external nonmaskable interrupt (NMI)
The processor automatically stacks its state on exception entry and unstacks this state on exception exit,
with no instruction overhead, providing low latency exception handling.
2.2.2.1
Level-Sensitive and Pulse Interrupts
The processor supports both level-sensitive and pulse interrupts. Pulse interrupts are also described as
edge-triggered interrupts.
A level-sensitive interrupt is held asserted until the peripheral deasserts the interrupt signal. Typically this
happens because the ISR accesses the peripheral, causing it to clear the interrupt request. A pulse
interrupt is an interrupt signal sampled synchronously on the rising edge of the processor clock. To ensure
the NVIC detects the interrupt, the peripheral must assert the interrupt signal for at least one clock cycle,
during which the NVIC detects the pulse and latches the interrupt.