ST62T45B STMicroelectronics, ST62T45B Datasheet - Page 28

• Manufacturer Part Number: ST62T45B
• Description: 8-BIT OTP/EPROM MCU
• Manufacturer: STMicroelectronics

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ST62T45B/E45B
IINTERRUPTS (Cont’d)
3.4.2 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
– The PC contents are stored in the first level of
– The normal interrupt lines are inhibited (NMI still
– 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-
– The source of the interrupt is found by polling the
– The interrupt is serviced.
– Return from interrupt (RETI)
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flags (or by the NMI flags).
the stack.
active).
terrupt service routine (normally on a software
stack).
interrupt flags (if more than one source is asso-
ciated with the same vector).
MCU
– Automatically the MCU switches back to the nor-
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
mal flag set (or the interrupt flag set) and pops
the previous PC value from the stack.
YES
NO
PROGRAM FLAGS
THE IN STRU CTION
INTER RU PT MASK
THE STAC KED PC
INS TRUCT ION
INS TRUC TION
IN STRU CTION
EXEC UTE
A RE TI
SELECT
W AS
FETCH
C LEAR
”POP”
?
?
YES
N O
YES
?
NO
NOR MAL MOD E?
AN IN TERRU PT RE QUEST
AN D INT ER RU PT MASK
IS THE COR E
ALREADY IN
C HEC K IF THER E IS
INTE RN AL MODE FLAG
PC INTO THE STAC K
IN TERR UPT VEC TOR
INTERR UP T MASK
LOAD PC FROM
PUSH THE
(FFC/ FFD)
SELECT
SET
VA000014