Functional Peripherals Description
83
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Cortex-M4F Peripherals
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.
When the processor enters the ISR, it automatically removes the pending state from the interrupt (see
for more information). For a level-sensitive interrupt, if the signal is not deasserted before
the processor returns from the ISR, the interrupt becomes pending again, and the processor must execute
its ISR again. As a result, the peripheral can hold the interrupt signal asserted until it no longer needs
servicing.
2.2.2.2
Hardware and Software Control of Interrupts
The Cortex-M4 latches all interrupts. A peripheral interrupt becomes pending for one of the following
reasons:
•
The NVIC detects that the interrupt signal is High and the interrupt is not active.
•
The NVIC detects a rising edge on the interrupt signal.
•
Software writes to the corresponding interrupt set-pending register bit, or to the Software Trigger
Interrupt Register (STIR) to make a Software-Generated Interrupt pending. See the INT bit in the
IPSR0 register or the STIR register.
A pending interrupt remains pending until one of the following:
•
The processor enters the ISR for the interrupt, changing the state of the interrupt from pending to
active. Then:
–
For a level-sensitive interrupt, when the processor returns from the ISR, the NVIC samples the
interrupt signal. If the signal is asserted, the state of the interrupt changes to pending, which might
cause the processor to immediately re-enter the ISR. Otherwise, the state of the interrupt changes
to inactive.
–
For a pulse interrupt, the NVIC continues to monitor the interrupt signal, and if this is pulsed the
state of the interrupt changes to pending and active. In this case, when the processor returns from
the ISR the state of the interrupt changes to pending, which might cause the processor to
immediately re-enter the ISR.
If the interrupt signal is not pulsed while the processor is in the ISR, when the processor returns
from the ISR the state of the interrupt changes to inactive.
•
Software writes to the corresponding interrupt clear-pending register bit
–
For a level-sensitive interrupt, if the interrupt signal is still asserted, the state of the interrupt does
not change. Otherwise, the state of the interrupt changes to inactive.
–
For a pulse interrupt, the state of the interrupt changes to inactive, if the state was pending or to
active, if the state was active and pending.
2.2.3 System Control Block (SCB)
The System Control Block (SCB) provides system implementation information and system control,
including configuration, control, and reporting of the system exceptions. See
for bit
description of System Control Block registers.