STR720 datasheet(67/401 Pages) STMICROELECTRONICS

67 / 401 page

STR720 - ENHANCED INTERRUPT CONTROLLER (EIC)

67/401

7.5 Programming considerations

Here are a few guideline steps on how to program the EIC’s registers in order to get the

controller up and running quickly. In the following, it is assumed to deal with standard

interrupts and that the user wants, for example, to detect an interrupt on channel #22, in which

the priority has been assigned to be 5.

First of all, it is necessary to assign the priority and the jump address data related to the

interrupt channel #22. Therefore:

■

set in field SIPL of SIR22 the binary “0101”, i.e. priority of 5 (it must be not 0 to have an IRQ

to be generated).

Two registers are involved to supply the channel interrupt vector data to the EIC controller

(IVR[31:16] and SIR22[31:16]:

■

write in SIR22[31:16], i.e. in the upper part of the SIR register related to channel #22, the

memory address offset (or the jump offset) where the Interrupt Service Routine, related to

interrupt channel #7, starts.

■

insert the base jump address (or the jump opcode) in the most significant half of the IVR

Finally, response to the interrupt must be enabled both at the global level and at the single

interrupt channel level. To do so these steps shall be followed:

■

set the IRQ_EN bit of ICR to 1

■

set bit # 22 of IER0 to 1

As far as the FIQ interrupts are concerned, since those interrupts do not have any

vectorization nor priority, only the first two steps above are involved. Supposing the user wants

to enable FIQ channel #1:

■

set the FIQ_EN bit of ICR to 1.

■

set bit #1 of FIE in FIR register to 1.

7.6 Application note

Every Interrupt Service Routine (ISR) should be composed by the following blocks of code.

■

A Header routine to enter ISR. It must be:

1) STMFD sp!,{r5,lr} The workspace r5 plus the current return address lr_irq

is pushed into the system stack.

2) MRS r5,spsr

Save the spsr into r5

3) STMFD sp!,{r5}

Save r5

4) MSR cpsr_c,#0x1F

Reenable IRQ, go into system mode

5) STMFD sp!,{lr}

Save lr_sys for the system mode

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