C501_1 Datasheet, PDF (88/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

Interrupt System
C501
7.4 External Interrupts
The external interrupts 0 and 1 can be programmed to be level-activated or negative-transition
activated by setting or clearing bit IT0, respectively in register TCON. If ITx = 0 (x = 0 or 1), external
interrupt x is triggered by a detected low level at the INTx pin. If ITx = 1, external interrupt x is
negative edge-triggered. In this mode, if successive samples of the INTx 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=1 then
requests the interrupt.
If the external interrupt 0 or 1 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.
The external timer 2 reload trigger interrupt request flag EXF2 will be activated by a negative
transition at pin P1.1/T2EX but only if bit EXEN2 is set.
Since the external interrupt pins are sampled once in each machine cycle, an input high or low
should be held for at least 6 oscillator periods to ensure sampling. lf 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 to ensure that the transition is recognized so that the
corresponding interrupt request flag will be set (see figure 7-27). The external interrupt request
flags will automatically be cleared by the CPU when the service routine is called.
Figure 7-27
External Interrupt Detection
Semiconductor Group
7-10