ST6200C datasheet(29/100 Pages) STMICROELECTRONICS

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6.8 INTERRUPT HANDLING PROCEDURE

The interrupt procedure is very similar to a call pro-

cedure, in fact 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.

The following list summarizes the interrupt proce-

dure:

When an interrupt request occurs, the following

actions are performed by the MCU automatically:

– The core switches from the normal flags to the

interrupt flags (or the NMI flags).

– The PC contents are stored in the top level of the

stack.

– The normal interrupt lines are inhibited (NMI still

active).

– The internal latch (if any) is cleared.

– The associated interrupt vector is loaded in the PC.

When an interrupt request occurs, the following

actions must be performed by the user software:

– User selected registers have to be saved within

the interrupt service routine (normally on a soft-

ware stack).

– The source of the interrupt must be determined

by polling the interrupt flags (if more than one

source is associated with the same vector).

– The RETI (RETurn from Interrupt) instruction

must end the interrupt service routine.

After the RETI instruction is executed, the MCU re-

turns to the main routine.

Caution: When a maskable interrupt occurs while

the ST6 core is in NORMAL mode and during the

execution of an “ldi IOR, 00h” instruction (disabling

all maskable interrupts): if the interrupt request oc-

curs 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.

6.8.1 Interrupt Response Time

This is defined as the time between the moment

when the Program Counter is loaded with the in-

terrupt vector and when the program has jump to

the interrupt subroutine and is ready to execute

the code. It depends on when the interrupt occurs

while the core is processing an instruction.

Figure 18. Interrupt Processing Flow Chart

Table 6. Interrupt Response Time

One CPU cycle is 13 external clock cycles thus 11

CPU cycles = 11 x (13 /8M) = 17.875 µs with an 8

MHz external quartz.

Minimum

6 CPU cycles

Maximum

11 CPU cycles

INSTRUCTION

FETCH

INSTRUCTION

EXECUTE

INSTRUCTION

WAS

THE INSTRUCTION

A RETI ?

ENABLE

MASKABLE INTERRUPTS

SELECT

NORMAL FLAGS

“POP”

THE STACKED PC

IS THERE AN

AN INTERRUPT REQUEST

AND INTERRUPT MASK?

SELECT

INTERRUPT FLAGS

PUSH THE

PC INTO THE STACK

LOAD PC FROM

INTERRUPT VECTOR

DISABLE

MASKABLE INTERRUPT

YES

IS THE CORE

ALREADY IN

NORMAL MODE?

YES

CLEAR

INTERNAL LATCH

*) If a latch is present on the interrupt source line

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