ST62E18C View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
ST62E18C
STMicroelectronics
'ST62E18C' PDF : 74 Pages View PDF
ST62T18C/E18C
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
NO
WAS
THE INS TRU CTION
A RE TI ?
YES
YES
?
IS THE CORE
ALREADY IN
NORMAL MODE?
NO
C LEAR
INT ERR UP T MASK
LOAD PC FROM
INT ERR UP T VEC TOR
(FFC /FF D)
SE T
INTER RU PT MASK
PUSH THE
PC IN TO THE STACK
SELECT
PROGRAM FLAGS
”POP”
THE STACK ED PC
S EL ECT
IN TER NA L MODE FLAG
NO
C HEC K IF THER E IS
?
AN IN TER RUP T R EQUEST
AN D INTE RRU PT MASK
Y ES
VA 000014
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