COP8-REF-AM datasheet(61/83 Pages) NSC

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18.0 Interrupts (Continued)

STPND flag will have the wrong state. This will allow

maskable interrupts to be acknowledged during the servicing

of the first Software Trap. To avoid problems such as this, the

user program should contain the Software Trap routine to

perform a recovery procedure rather than a return to normal

execution.

Under normal conditions, the STPND flag is reset by a

RPND instruction in the Software Trap service routine. If a

programming error or hardware condition (brownout, power

supply glitch, etc.) sets the STPND flag without providing a

way for it to be cleared, all other interrupts will be locked out.

To alleviate this condition, the user can use extra RPND

instructions in the main program and in the Watchdog ser-

vice routine (if present). There is no harm in executing extra

RPND instructions in these parts of the program.

18.4.2.1 Programming Example: External Interrupt

PSW

=00EF

CNTRL

=00EE

RBIT

0,PORTGC

RBIT

0,PORTGD

; G0 pin configured Hi-Z

SBIT

IEDG, CNTRL

; Ext interrupt polarity; falling edge

SBIT

GIE, PSW

; Set the GIE bit

SBIT

EXEN, PSW

; Enable the external interrupt

WAIT:

WAIT

; Wait for external interrupt

.=0FF

; The interrupt causes a

VIS

; branch to address 0FF

; The VIS causes a branch to

; interrupt vector table

.=01FA

; Vector table (within 256 byte

.ADDRW SERVICE

; of VIS inst.) containing the ext

; interrupt service routine

20006334

FIGURE 33. VIS Flow Chart

COP8-REF-AM Datasheet(PDF) 61 Page - National Semiconductor (TI)

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