UM11029
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User manual
Rev. 1.0 — 16 June 2017
111 of 515
NXP Semiconductors
UM11029
Chapter 8: LPC84x System configuration (SYSCON)
8.6.40 IRQ latency register
The IRQLATENCY register is an eight-bit register which specifies the minimum number of
cycles (0-255) permitted for the system to respond to an interrupt request. The intent of
this register is to allow the user to select a trade-off between interrupt response time and
determinism.
Setting this parameter to a very low value (e.g. zero) will guarantee the best possible
interrupt performance but will also introduce a significant degree of uncertainty and jitter.
Requiring the system to always take a larger number of cycles (whether it needs it or not)
will reduce the amount of uncertainty but may not necessarily eliminate it.
Theoretically, the ARM Cortex-M0+ core should always be able to service an interrupt
request within 15 cycles. However, system factors external to the CPU, such as bus
latencies or peripheral response times, can increase the time required to complete a
previous instruction before an interrupt can be serviced. Therefore, accurately specifying
a minimum number of cycles that will ensure determinism will depend on the application.
The default setting for this register is 0x010.
8.6.41 NMI source selection register
The NMI source selection register selects a peripheral interrupt as source for the NMI
interrupt of the ARM Cortex-M0+ core. For a list of all peripheral interrupts and their IRQ
numbers see
. For a description of the NMI functionality, see
.
Remark:
When you want to change the interrupt source for the NMI, you must first disable
the NMI source by setting bit 31 in this register to 0. Then change the source by updating
the IRQN bits and re-enable the NMI source by setting bit 31 to 1.
Remark:
If the NMISRC register is used to select an interrupt as the source of
Non-Maskable interrupts, and the selected interrupt is enabled, one interrupt request can
result in both a Non-Maskable and a normal interrupt. This can be avoided by disabling
the normal interrupt in the NVIC.
Table 165. IRQ latency register (IRQLATENCY, address 0x4004 8170) bit description
Bit
Symbol
Description
Reset
value
7:0
LATENCY
8-bit latency value
0x010
31:8
-
Reserved
-
Table 166. NMI source selection register (NMISRC, address 0x4004 8174) bit description
Bit
Symbol Description
Reset
value
4:0
IRQN
The IRQ number of the interrupt that acts as the Non-Maskable Interrupt
(NMI) if bit 31 is 1. See
for the list of interrupt sources and their
IRQ numbers.
0
30:5
-
Reserved
-
31
NMIEN
Write a 1 to this bit to enable the Non-Maskable Interrupt (NMI) source
selected by bits 4:0.
0