UPD75518 NEC, UPD75518 Datasheet - Page 137

• Manufacturer Part Number: UPD75518
• Description: 4 BIT SINGLE-CHIP MICROCOMPUTER
• Manufacturer: NEC

5.5 VECTOR ADDRESS SHARE INTERRUPT PROCESSING
(1) When using only one interrupt source
(2) When using both interrupt sources
Since interrupt sources INTBT and INT4 share the vector table, the following two cases must be considered.
The interrupt enable flag corresponding to the required interrupt source of the two interrupt sources
sharing the vector table is set and the other interrupt enable flag is cleared. In this case, the enabled
interrupt source (IE
interrupt request flag is reset.
The interrupt enable flags corresponding to the two interrupt sources are set. In this case, the logical and
of the interrupt request flags corresponding to the two interrupt sources is an interrupt request.
Even if one or both of the interrupt request flags are set and an interrupt request is accepted, neither of
the interrupt request flags is reset.
The interrupt service routine must therefore judge which interrupt source caused an interrupt. This is done
by executing a DI instruction at the beginning of the interrupt service routine and checking the interrupt
request flags with an SKTCLR instruction.
Remark When only one interrupt is enabled, its interrupt source can be clearly identified, so that the
interrupt request flag is reset by hardware at acceptance of the interrupt. However, when both
interrupts are enabled, the interrupt source cannot be identified, so that the interrupt request
flags cannot be reset by hardware. For this reason, the interrupt request flags are checked by
software to determine the interrupt source.
= 1) issues an interrupt request. If this request is accepted, the corresponding
PD75518(A)
137