PIC16C63A/65B/73B/74B
13.5 Interrupts
Note: If an interrupt occurs while the Global Inter-
rupt Enable (GIE) bit is being cleared, the
GIE bit may unintentionally be re-enabled
by the user’s Interrupt Service Routine (the
RETFIE instruction). The events that
would cause this to occur are:
The Interrupt Control register (INTCON) records indi-
vidual interrupt requests in flag bits. It also has individ-
ual and global interrupt enable bits.
Note: Individual interrupt flag bits are set, regard-
less of the status of their corresponding
mask bit, or the GIE bit.
1. An instruction clears the GIE bit while an
interrupt is acknowledged.
A global interrupt enable bit, GIE (INTCON<7>),
enables (if set) all unmasked interrupts, or disables (if
cleared) all interrupts. When bit GIE is enabled, and an
interrupt’s flag bit and mask bit are set, the interrupt will
vector immediately. Individual interrupts can be dis-
abled through their corresponding enable bits in vari-
ous registers. Individual interrupt bits are set,
regardless of the status of the GIE bit. The GIE bit is
cleared on RESET.
2. The program branches to the interrupt
vector and executes the Interrupt
Service Routine.
3. The Interrupt Service Routine completes
the execution of the RETFIEinstruction.
This causes the GIE bit to be set
(enables interrupts), and the program
returns to the instruction after the one
which was meant to disable interrupts.
The “return from interrupt” instruction, RETFIE, exits
the interrupt routine, as well as sets the GIE bit, which
re-enables interrupts.
Perform the following to ensure that inter-
rupts are globally disabled:
LOOP BCF
INTCON, GIE ; Disable global
; interrupt bit
The RB0/INT pin interrupt, the RB port change interrupt
and the TMR0 overflow interrupt flags are contained in
the INTCON register.
BTFSC INTCON, GIE ; Global interrupt
; disabled?
GOTO LOOP
:
; NO, try again
; Yes, continue
; with program
; flow
The peripheral interrupt flags are contained in the spe-
cial function registers PIR1 and PIR2. The correspond-
ing interrupt enable bits are contained in special
function registers PIE1 and PIE2 and the peripheral
interrupt enable bit is contained in special function reg-
ister INTCON.
When an interrupt is responded to, the GIE bit is
cleared to disable any further interrupt, the return
address is pushed onto the stack, and the PC is loaded
with 0004h. Once in the Interrupt Service Routine, the
source(s) of the interrupt can be determined by polling
the interrupt flag bits. The interrupt flag bit(s) must be
cleared in software before re-enabling interrupts to
avoid recursive interrupts.
For external interrupt events, such as the INT pin or
PORTB change interrupt, the interrupt latency will be
three or four instruction cycles. The exact latency
depends when the interrupt event occurs. The latency
is the same for one or two cycle instructions. Individual
interrupt flag bits are set, regardless of the status of
their corresponding mask bit, PEIE bit, or the GIE bit.
2000 Microchip Technology Inc.
DS30605C-page 93
MICROCHIP [ MICROCHIP ]
