ST63142 datasheet(25/86 Pages) STMICROELECTRONICS

25 / 86 page

MCU Initialization Sequence

When a reset occurs the stack is reset to the pro-

gram counter, the PC is loaded with the address of

the reset vector (located in the program ROM at

addresses FFEh & FFFh). A jump instruction to the

beginning of the program has to be written into

these locations. After a reset a NMI is automat-

ically activated so that the core is in non-maskable

interrupt mode to prevent false or ghost interrupts

during the restart phase. Therefore the restart rou-

tine should be terminated by a RETI instruction to

switch to normal mode and enable interrupts. If no

pending interrupt is present at the end of the reset

routine the ST631xx will continue with the instruc-

tion after the RETI; otherwise the pending interrupt

will be serviced.

RESET

IS RESET

STILL PRESENT ?

YES

VA000427

NMI MASK SET

INT LATCH CLEARED

( IF PRESENT )

SELECT

NMI MODE FLAGS

PUT FFEh

ON ADDRESS BUS

LOAD PC

FROM RESET LOCATIONS

FFE / FFF

FETCH INSTRUCTION

Figure 21. Reset & Interrupt Processing

Flow-Chart

RESET VECTOR

INITIALIZATION

ROUTINE

JP: 2 BYTES/4 CYCLES

RETI: 1BYTES/2 CYCLES

RETI

VA 000181

RESET

Figure 22. Restart Initialization Program

Flow-Chart

RESET (Continued)

ST63140,142,126,156

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