Philips Semiconductors Product data
P89C60X2/61X2
80C51 8-bit Flash microcontroller family
64KB Flash, 512B/1024B RAM
2003 Sep 11
41
. . . .
C1 C2 C3 C4 C5
. . . .
. . . .
Interrupts
Are Polled
Long Call to
Interrupt
Vector Address
Interrupt Routine
ε
Interrupt
Goes
Active
. . . . . . . . .
Interrupt
Latched
This is the fastest possible response when C2 is the final cycle of an instruction other than RETI or an access to IE or IP.
S5P2 S6
. . . . . . . . .
SU00546
Figure 27. Interrupt Response Timing Diagram
The polling cycle/LCALL sequence is illustrated in Figure 27.
Note that if an interrupt of higher priority level goes active prior to
S5P2 of the machine cycle labeled C3 in Figure 27, then in
accordance with the above rules it will be vectored to during C5 and
C6, without any instruction of the lower priority routine having been
executed.
Thus the processor acknowledges an interrupt request by executing
a hardware-generated LCALL to the appropriate servicing routine. In
some cases it also clears the flag that generated the interrupt, and in
other cases it doesn’t. It never clears the Serial Port flag. This has to
be done in the user’s software. It clears an external interrupt flag
(IE0 or IE1) only if it was transition-activated. The
hardware-generated LCALL pushes the contents of the Program
Counter on to the stack (but it does not save the PSW) and reloads
the PC with an address that depends on the source of the interrupt
being vectored to, as shown in Table 10.
Execution proceeds from that location until the RETI instruction is
encountered. The RETI instruction informs the processor that this
interrupt routine is no longer in progress, then pops the top two
bytes from the stack and reloads the Program Counter. Execution of
the interrupted program continues from where it left off.
Note that a simple RET instruction would also have returned
execution to the interrupted program, but it would have left the
interrupt control system thinking an interrupt was still in progress,
making future interrupts impossible.
External Interrupts
The external sources can be programmed to be level-activated or
transition-activated by setting or clearing bit IT1 or IT0 in Register
TCON. If ITx = 0, external interrupt x is triggered by a detected low
at the INT
x pin. If ITx = 1, external interrupt x is edge triggered. In
this mode if successive samples of the INT
x pin show a high in one
cycle and a low in the next cycle, interrupt request flag IEx in TCON
is set. Flag bit IEx then requests the interrupt.
Since the external interrupt pins are sampled once each machine
cycle, an input high or low should hold for at least 12 oscillator
periods to ensure sampling. If the external interrupt is
transition-activated, the external source has to hold the request pin
high for at least one cycle, and then hold it low for at least one cycle.
This is done to ensure that the transition is seen so that interrupt
request flag IEx will be set. IEx will be automatically cleared by the
CPU when the service routine is called.
If the external interrupt is level-activated, the external source has to
hold the request active until the requested interrupt is actually
generated. Then it has to deactivate the request before the interrupt
service routine is completed, or else another interrupt will be
generated.
Response Time
The INT0
and INT1 levels are inverted and latched into IE0 and IE1
at S5P2 of every machine cycle. The values are not actually polled
by the circuitry until the next machine cycle. If a request is active
and conditions are right for it to be acknowledged, a hardware
subroutine call to the requested service routine will be the next
instruction to be executed. The call itself takes two cycles. Thus, a
minimum of three complete machine cycles elapse between
activation of an external interrupt request and the beginning of
execution of the first instruction of the service routine. Figure 27
shows interrupt response timings.
A longer response time would result if the request is blocked by one
of the 3 previously listed conditions. If an interrupt of equal or higher
priority level is already in progress, the additional wait time obviously
depends on the nature of the other interrupt’s service routine. If the
instruction in progress is not in its final cycle, the additional wait time
cannot be more the 3 cycles, since the longest instructions (MUL
and DIV) are only 4 cycles long, and if the instruction in progress is
RETI or an access to IE or IP, the additional wait time cannot be
more than 5 cycles (a maximum of one more cycle to complete the
instruction in progress, plus 4 cycles to complete the next instruction
if the instruction is MUL or DIV).
Thus, in a single-interrupt system, the response time is always more
than 3 cycles and less than 9 cycles.
As previously mentioned, the derivatives described in this data
sheet have a four-level interrupt structure. The corresponding
registers are IE, IP and IPH. (See Figures 24, 25, and 26.) The IPH
(Interrupt Priority High) register makes the four-level interrupt
structure possible.
The function of the IPH SFR is simple and when combined with the
IP SFR determines the priority of each interrupt. The priority of each
interrupt is determined as shown in the following table:
PRIORITY BITS
IPH.x IP.x
0 0 Level 0 (lowest priority)
0 1 Level 1
1 0 Level 2
1 1 Level 3 (highest priority)
