ST62T35B datasheet(26/82 Pages) STMICROELECTRONICS

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ST62T35B/E35B

IINTERRUPTS (Cont’d)

3.3.2 Interrupt Procedure

The interrupt procedure is very similar to a call

procedure, indeed the user can consider the inter-

rupt as an asynchronous call procedure. As this is

an asynchronous event, the user cannot know the

context and the time at which it occurred. As a re-

sult, the user should save all Data space registers

which may be used within the interrupt routines.

There are separate sets of processor flags for nor-

mal, interrupt and non-maskable interrupt modes,

which are automatically switched and so do not

need to be saved.

The following list summarizes the interrupt proce-

dure:

MCU

– The interrupt is detected.

– The C and Z flags are replaced by the interrupt

flags (or by the NMI flags).

– The PC contents are stored in the first level of

the stack.

– The normal interrupt lines are inhibited (NMI still

active).

– The first internal latch is cleared.

– The associated interrupt vector is loaded in thePC.

WARNING: In some circumstances, when a

maskable interrupt occurs while the ST6 core is in

NORMAL mode and especially during the execu-

tion of an ”ldi IOR, 00h” instruction (disabling all

maskable interrupts): if the interrupt arrives during

the first 3 cycles of the ”ldi” instruction (which is a

4-cycle instruction) the core will switch to interrupt

mode BUT the flags CN and ZN will NOT switch to

the interrupt pair CI and ZI.

User

– User selected registers are saved within the in-

terrupt service routine (normally on a software

stack).

– The source of the interrupt is found by polling the

interrupt flags (if more than one source is asso-

ciated with the same vector).

– The interrupt is serviced.

– Return from interrupt (RETI)

MCU

– Automatically the MCU switches back to the nor-

mal flag set (or the interrupt flag set) and pops

the previous PC value from the stack.

The interrupt routine usually begins by the identi-

fying the device which generated the interrupt re-

quest (by polling). The user should save the regis-

ters which are used within the interrupt routine in a

software stack. After the RETI instruction is exe-

cuted, the MCU returns to the main routine.

Figure 16. Interrupt Processing Flow Chart

I NS TRUC TION

FETCH

IN STRU CTION

EXEC UTE

INS TRUCT ION

WAS

THE IN STRU CTION

ARE TI

CLEAR

I NTER RU PT MASK

SELECT

PROGRAM FLAGS

”POP”

THE STAC KED PC

C HEC K IF THER E IS

AN IN TERRU PT RE QUEST

AN D INT ER RU PT MASK

SELECT

INTE RN AL MODE FLAG

PUSH THE

PC I NTO THE STAC K

LOAD PC FROM

IN TERR UPT VEC TOR

(FFC/ FFD)

SET

I NTERR UP T MASK

YES

I S THE COR E

ALREADY IN

NOR MAL MOD E?

VA000014

YES

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