ST6208CB3 datasheet(27/70 Pages) STMICROELECTRONICS

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ST62T08C/T09C ST62T10C/T20C/E20C

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.

– The associated interrupt vector is loaded in the 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

INSTRUCTION

FETCH

INSTRUCTION

EXECUTE

INSTRUCTION

WAS

THE INSTRUCTION

A RETI

CLEAR

INTERRUPT MASK

SELECT

PROGRAM FLAGS

"POP"

THE STACKED PC

CHECK IF THERE IS

AN INTERRUPT REQUEST

AND INTERRUPT MASK

SELECT

INTERNAL MODE FLAG

PUSH THE

PC INTO THE STACK

LOAD PC FROM

INTERRUPT VECTOR

(FFC/FFD)

SET

INTERRUPT MASK

YES

IS THE CORE

ALREADY IN

NORMAL MODE?

VA000014

YES

115

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