ST7LITE49M datasheet(52/188 Pages) STMICROELECTRONICS

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Interrupts

ST7LITE49M

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8.2.1

Servicing pending interrupts

As several interrupts can be pending at the same time, the interrupt to be taken into account

is determined by the following two-step process:

●

The highest software priority interrupt is serviced,

●

If several interrupts have the same software priority then the interrupt with the highest

hardware priority is serviced first.

Figure 21 describes this decision process.

Figure 21.

Priority decision process

When an interrupt request is not serviced immediately, it is latched and then processed

when its software priority combined with the hardware priority becomes the highest one.

Note:

The hardware priority is exclusive while the software one is not. This allows the previous

process to succeed with only one interrupt.

RESET and TRAP can be considered as having the highest software priority in the decision

process.

8.2.2

Interrupt vector sources

Two interrupt source types are managed by the ST7 interrupt controller: the non-maskable

type (RESET, TRAP) and the maskable type (external or from internal peripherals).

Non-maskable sources

These sources are processed regardless of the state of the I1 and I0 bits of the CC register

(see Figure 20). After stacking the PC, X, A and CC registers (except for RESET), the

corresponding vector is loaded in the PC register and the I1 and I0 bits of the CC are set to

disable interrupts (level 3). These sources allow the processor to exit Halt mode.

●

TRAP (non maskable software interrupt)

This software interrupt is serviced when the TRAP instruction is executed. It will be

serviced according to the flowchart in Figure 20.

●

RESET

The RESET source has the highest priority in the ST7. This means that the first current

routine has the highest software priority (level 3) and the highest hardware priority.

See the RESET chapter for more details.

PENDING

SOFTWARE

Different

INTERRUPTS

Same

HIGHEST HARDWARE

PRIORITY SERVICED

PRIORITY

HIGHEST SOFTWARE

PRIORITY SERVICED

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