UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 20 December 2013 766 of 841
NXP Semiconductors
UM10360
Chapter 34: Appendix: Cortex-M3 user guide
When the processor enters the ISR, it automatically removes the pending state from the
interrupt, see Section 34.4.2.9.1
. 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. This means that the peripheral can
hold the interrupt signal asserted until it no longer needs servicing.
34.4.2.9.1 Hardware and software control of interrupts
The Cortex-M3 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, see Table 648,
or to the STIR to make an SGI pending, see Table 652
.
A pending interrupt remains pending until one of the following:
• The processor enters the ISR for the interrupt. This changes 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, state of the interrupt changes to:
– inactive, if the state was pending
– active, if the state was active and pending.
34.4.2.10 NVIC design hints and tips
Ensure software uses correctly aligned register accesses. The processor does not
support unaligned accesses to NVIC registers. See the individual register descriptions for
the supported access sizes.
A interrupt can enter pending state even it is disabled.
Before programming VTOR to relocate the vector table, ensure the vector table entries of
the new vector table are setup for fault handlers, NMI and all enabled exception like
interrupts. For more information see Table 658
.
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