UPD784938 Datasheet, PDF (595/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 23 INTERRUPT FUNCTIONS
Cautions 1. When an interrupt related register is polled using a BF instruction, etc., the branch destination
of that BR instruction, etc., should not be that instruction. If a program is written in which a branch
is made to that instruction itself, all interrupts and macro service requests will be held pending
until a condition whereby a branch is not made by that instruction arises.
Bad Example
LOOP : BF PIC0.7, $LOOP
×××
All interrupts and macro service requests are held pending until
PIC0.7 is 1.
← Interrupts and macro service requests are not serviced until
after execution of the instruction following the BF instruction.
Good Example (1)
LOOP : NOP
BF PIC0.7, $LOOP
← Interrupts and macro service requests are serviced after execution
of the NOP instruction, so that interrupts are never held
pending for a long period.
Good Example (2)
LOOP : BT PIC0.7, $NEXT
BR $LOOP
NEXT :
Using a BTCLR instruction instead of a BT instruction has the
advantage that the flag is cleared (to 0) automatically.
← Interrupts and macro service requests are serviced after
execution of the BR instruction, so that interrupts are never held
pending for a long period.
2. For a similar reason, if problems are caused by a long pending period for interrupts and macro
service when instructions to which the above applies are used in succession, a time at which
interrupts and macro service requests can be acknowledged should be provided by inserting an
NOP instruction, etc., in the series of instructions.
Preliminary User’s Manual U13987EJ1V0UM00
595