UM10147_2
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 02 — 28 April 2008
34 of 134
NXP Semiconductors
UM10147
P89LPC952/954 User manual
Each interrupt source can be individually enabled or disabled by setting or clearing a bit in
the interrupt enable registers IEN0 or IEN1. The IEN0 register also contains a global
enable bit, EA, which enables all interrupts.
Each interrupt source can be individually programmed to one of four priority levels by
setting or clearing bits in the interrupt priority registers IP0, IP0H, IP1, and IP1H. An
interrupt service routine in progress can be interrupted by a higher priority interrupt, but
not by another interrupt of the same or lower priority. The highest priority interrupt service
cannot be interrupted by any other interrupt source. If two requests of different priority
levels are received simultaneously, the request of higher priority level is serviced.
If requests of the same priority level are pending at the start of an instruction cycle, an
internal polling sequence determines which request is serviced. This is called the
arbitration ranking. Note that the arbitration ranking is only used for pending requests of
the same priority level.
summarizes the interrupt sources, flag bits, vector
addresses, enable bits, priority bits, arbitration ranking, and whether each interrupt may
wake-up the CPU from a Power-down mode.
4.1 Interrupt priority structure
There are four SFRs associated with the four interrupt levels: IP0, IP0H, IP1, IP1H. Every
interrupt has two bits in IPx and IPxH (x = 0, 1) and can therefore be assigned to one of
four levels, as shown in
.
The P89LPC952/954 has two external interrupt inputs in addition to the Keypad Interrupt
function. The two interrupt inputs are identical to those present on the standard 80C51
microcontrollers.
These external interrupts can be programmed to be level-triggered or edge-triggered by
clearing or setting bit IT1 or IT0 in Register TCON. If ITn = 0, external interrupt n is
triggered by a low level detected at the INTn pin. If ITn = 1, external interrupt n is edge
triggered. In this mode if consecutive samples of the INTn pin show a high level in one
cycle and a low level in the next cycle, interrupt request flag IEn in TCON is set, causing
an interrupt request.
Since the external interrupt pins are sampled once each machine cycle, an input high or
low level should be held for at least one machine cycle to ensure proper sampling. If the
external interrupt is edge-triggered, the external source has to hold the request pin high
for at least one machine cycle, and then hold it low for at least one machine cycle. This is
to ensure that the transition is detected and that interrupt request flag IEn is set. IEn is
automatically cleared by the CPU when the service routine is called.
Table 21.
Interrupt priority level
Priority bits
IPxH
IPx
Interrupt priority level
0
0
Level 0 (lowest priority)
0
1
Level 1
1
0
Level 2
1
1
Level 3
