ST72324J2 datasheet(160/164 Pages) STMICROELECTRONICS

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KNOWN LIMITATIONS (Cont’d)

To avoid this, a semaphore is set to '1' before

checking the level change. The semaphore is

changed to level '0' inside the interrupt routine.

When a level change is detected, the semaphore

status is checked and if it is '1' this means that the

last interrupt has been missed. In this case, the in-

terrupt routine is invoked with the call instruction.

There is another possible case, that is, if writing to

PxOR or PxDDR is done with global interrupts dis-

abled (interrupt mask bit set). In this case, the

semaphore is changed to '1' when the level

change is detected. Detecting a missed interrupt is

done after the global interrupts are enabled (inter-

rupt mask bit reset) and by checking the status of

the semaphore. If it is '1' this means that the last

interrupt was missed and the interrupt routine is in-

voked with the call instruction.

To implement the workaround, the following soft-

ware sequence is to be followed for writing into the

PxOR/PxDDR registers. The example is for Port

PF1 with falling edge interrupt sensitivity. The soft-

ware sequence is given for both cases (global in-

terrupt disabled/enabled).

Case 1: Writing to PxOR or PxDDR with Global In-

terrupts Enabled:

LD A,#01

LD sema,A

; set the semaphore to '1'

LD A,PFDR

AND A,#02

LD X,A

; store the level before writing to

PxOR/PxDDR

LD A,#$90

LD PFDDR,A ; Write to PFDDR

LD A,#$ff

LD PFOR,A

; Write to PFOR

LD A,PFDR

AND A,#02

LD Y,A

; store the level after writing to

PxOR/PxDDR

LD A,X

; check for falling edge

cp A,#02

jrne OUT

TNZ Y

jrne OUT

LD A,sema

; check the semaphore status if

edge is detected

CP A,#01

jrne OUT

call call_routine; call the interrupt routine

OUT:LD A,#00

LD sema,A

.call_routine

; entry to call_routine

PUSH A

PUSH X

PUSH CC

.ext1_rt

; entry to interrupt routine

LD A,#00

LD sema,A

IRET

Case 2: Writing to PxOR or PxDDR with Global In-

terrupts Disabled:

SIM

; set the interrupt mask

LD A,PFDR

AND A,#$02

LD X,A

; store the level before writing to

PxOR/PxDDR

LD A,#$90

LD PFDDR,A ; Write into PFDDR

LD A,#$ff

LD PFOR,A

; Write to PFOR

LD A,PFDR

AND A,#$02

LD Y,A

; store the level after writing to

PxOR/PxDDR

LD A,X

; check for falling edge

cp A,#$02

jrne OUT

TNZ Y

jrne OUT

LD A,#$01

LD sema,A

; set the semaphore to '1' if edge is

detected

RIM

; reset the interrupt mask

LD A,sema

; check the semaphore status

CP A,#$01

jrne OUT

call call_routine; call the interrupt routine

RIM

OUT:

RIM

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