AMD Am186TMER, Am188TMER user manual Special Fully Nested Mode, Operation in a Polled Environment

8.2.3Special Fully Nested Mode

Special fully nested mode is entered by setting the SFNM bit in the INT0 or INT1 control registers. (See section 8.3.1 on page 8-14.)It enables complete nesting with external 82C59A masters or multiple interrupts from the same external interrupt pin when not in Cascade mode. In this case, the ISRs must be re-entrant.

In fully nested mode, an interrupt request from an interrupt source is not recognized when the in-service bit for that source is set. In this case, if more than one interrupt source is connected to an external interrupt controller, all of the interrupts go through the same Am186ER or Am188ER microcontroller interrupt request pin. As a result, if the external interrupt controller receives a higher-priority interrupt, its interrupt is not recognized by the microcontroller until the in-service bit is reset.

In special fully nested mode, the microcontroller’s interrupt controller allows the processor to take interrupts from an external pin regardless of the state of the in-service bit for an interrupt source. This allows multiple interrupts from a single pin. An in-service bit continues to be set, however, to inhibit interrupts from other lower-priority Am186ER or Am188ER microcontroller interrupt sources.

In special fully nested mode with Cascade mode, when a write is issued to the EOI Register at the end of the interrupt service routine, software polling of the IS Register in the external master 82C59A must determine if there is more than one IS bit set. If so, the IS bit in the microcontroller remains active and the next ISR is entered.

8.2.4Operation in a Polled Environment

To allow reading of the Poll Register information without setting the indicated in-service bit, the Am186ER and Am188ER microcontrollers provide a Poll Status Register (Figure 8-15)in addition to the Poll Register. Poll Register information is duplicated in the Poll Status Register, but the Poll Status Register can be read without setting the associated in-service bit. These registers are located in two adjacent memory locations in the peripheral control block.

The interrupt controller can be used in polled mode if interrupts are not desired. When polling, interrupts are disabled and software polls the interrupt controller as required. The interrupt controller is polled by reading the Poll Status Register (Figure 8-15). Bit 15 in the Poll Status Register indicates to the processor that an interrupt of high enough priority is requesting service. Bits 4–0 indicate to the processor the interrupt type of the highest priority source requesting service. After determining that an interrupt is pending, software reads the Poll Register (rather than the Poll Status Register), which causes the in-service bit of the highest priority source to be set.

8.2.5End-of-Interrupt Write to the EOI Register

A program must write to the EOI Register to reset the in-service (IS) bit when an interrupt service routine is completed. There are two types of writes to the EOI Register—specific EOI and non-specific EOI (see section 8.3.14 on page 8-28).

Non-specific EOI does not specify which IS bit is to be reset. Instead, the interrupt controller automatically resets the IS bit of the highest priority source with an active service routine.

Specific EOI requires the program to send the interrupt type to the interrupt controller to indicate the source IS bit that is to be reset. Specific reset is applicable when interrupt nesting is possible or when the highest priority IS bit that was set does not belong to the service routine in progress.

8-12 Interrupt Control Unit