C517A_99 Datasheet, PDF (178/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Interrupt System
C517A
7.1.2 Interrupt Request / Control Flags
The request flags for the different interrupt sources are located in several special function registers.
This section describes the locations and meanings of these interrupt request flags in detail.
The external interrupts 0 and 1 (P3.2/INT0 and P3.3/INT1) can each be either level-activated or
negative transition-activated, depending on bits IT0 and IT1 in SFR TCON. The flags that actually
generate these interrupts are bits IE0 and lE1 in SFR TCON. When an external interrupt is
generated, the flag that generated this interrupt is cleared by the hardware when the service routine
is vectored to, but only if the interrupt was transition-activated. lf the interrupt was level-activated,
then the requesting external source directly controls the request flag, rather than the on-chip
hardware.
The timer 0 and timer 1 interrupts are generated by TF0 and TF1 in register TCON, which are set
by a rollover in their respective timer/counter registers (exception is timer 0 in mode 3). When a
timer interrupt is generated, the flag that generated it is cleared by the on-chip hardware when the
service routine is vectored to.
Special Function Register TCON (Address 88H)
Reset Value : 00H
MSB
Bit No. 8F_H
8E_H 8D_H
8C_H
8B_H 8A_H
LSB
89_H 88_H
88H TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0 TCON
The shaded bits are not used for interrupt purposes.
Bit
Function
TF1
Timer 1 overflow flag
Set by hardware on timer/counter 1 overflow. Cleared by hardware when
processor vectors to interrupt routine.
TF0
Timer 0 overflow flag
Set by hardware on timer/counter 0 overflow. Cleared by hardware
when processor vectors to interrupt routine.
IE1
External interrupt 1 request flag
Set by hardware. Cleared by hardware when processor vectors to interrupt routine
(if IT1 = 1) or by hardware (if IT1 = 0).
IT1
External interrupt 1 level/edge trigger control flag
If IT1 = 0, level triggered external interrupt 1 is selected.
If IT1 = 1, negative edge triggered external interrupt 1 is selected.
IE0
External interrupt 0 request flag
Set by hardware. Cleared by hardware when processor vectors to interrupt routine
(if IT0 = 1) or by hardware (if IT0 = 0).
IT0
External interrupt 0 level/edge trigger control flag
If IT0 = 0, level triggered external interrupt 0 is selected.
If IT0 = 1, negative edge triggered external interrupt 0 is selected.
Semiconductor Group
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