ST6255CB1 datasheet(32/86 Pages) STMICROELECTRONICS

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ST62T55C ST62T65C/E65C

INTERRUPTS (Cont’d)

3.4.2 Interrupt Procedure

The interrupt procedure is very similar to a call pro-

cedure, indeed the user can consider the interrupt

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.

– Theassociated interrupt vectoris loaded inthe PC.

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 associ-

ated 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 identify-

ing 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 20. Interrupt Processing Flow Chart

INS TRU CTION

FETCH

INS TRU CTION

EXEC UTE

IN STRUC TION

WAS

THE INS TRU CTION

ARE TI ?

C LEAR

INT ERR UP T MASK

SELECT

PROGRAM FLAGS

”POP”

THE STACK ED PC

C HEC K IF THER E IS

AN IN TER RUP T R EQUEST

AN D INTE RRU PT MASK

SELECT

IN TER NA L MODE FLAG

PUSH THE

PC IN TO THE STACK

LOAD PC FROM

INT ERR UP T VEC TOR

(FFC/FFD)

SET

INTER RU PT MASK

YES

IS THE CORE

ALREADY I N

NORMAL MODE?

VA000014

YES

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