PIC17C756A MICROCHIP [Microchip Technology], PIC17C756A Datasheet - Page 39

• Manufacturer Part Number: PIC17C756A
• Description: High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

6.4
Global Interrupt Disable bit, GLINTD (CPUSTA<4>),
enables all unmasked interrupts (if clear), or disables
all interrupts (if set). Individual interrupts can be dis-
abled through their corresponding enable bits in the
INTSTA register. Peripheral interrupts need either the
global peripheral enable PEIE bit disabled, or the spe-
cific peripheral enable bit disabled. Disabling the
peripherals via the global peripheral enable bit, dis-
ables all peripheral interrupts. GLINTD is set on
RESET (interrupts disabled).
The RETFIE instruction clears the GLINTD bit while
forcing the Program Counter (PC) to the value loaded
at the Top-of-Stack.
When an interrupt is responded to, the GLINTD bit is
automatically set to disable any further interrupt, the
return address is pushed onto the stack and the PC is
loaded with the interrupt vector. There are four interrupt
vectors which help reduce interrupt latency.
The peripheral interrupt vector has multiple interrupt
sources. Once in the peripheral Interrupt Service Rou-
tine, the source(s) of the interrupt can be determined by
polling the interrupt flag bits. The peripheral interrupt
flag bit(s) must be cleared in software before re-
enabling interrupts to avoid continuous interrupts.
The PIC17C7XX devices have four interrupt vectors.
These vectors and their hardware priority are shown in
Table 6-1. If two enabled interrupts occur “at the same
time”, the interrupt of the highest priority will be ser-
viced first. This means that the vector address of that
interrupt will be loaded into the program counter (PC).
TABLE 6-1:
0008h
0010h
0018h
0020h
Address
2000 Microchip Technology Inc.
Note 1: Individual interrupt flag bits are set, regard-
Interrupt Operation
2: Before disabling any of the INTSTA enable
External Interrupt on RA0/
INT pin (INTF)
TMR0 Overflow Interrupt
(T0IF)
External Interrupt on T0CKI
(T0CKIF)
Peripherals (PEIF)
less of the status of their corresponding
mask bit or the GLINTD bit.
bits, the GLINTD bit should be set
(disabled).
INTERRUPT VECTORS/
PRIORITIES
Vector
1 (Highest)
4 (Lowest)
Priority
2
3
6.5
The external interrupt on the RA0/INT pin is edge trig-
gered. Either the rising edge if the INTEDG bit
(T0STA<7>) is set, or the falling edge if the INTEDG bit
is clear. When a valid edge appears on the RA0/INT
pin, the INTF bit (INTSTA<4>) is set. This interrupt can
be disabled by clearing the INTE control bit
(INTSTA<0>). The INT interrupt can wake the proces-
sor from SLEEP. See Section 17.4 for details on
SLEEP operation.
6.6
The external interrupt on the RA1/T0CKI pin is edge
triggered. Either the rising edge if the T0SE bit
(T0STA<6>) is set, or the falling edge if the T0SE bit is
clear. When a valid edge appears on the RA1/T0CKI
pin, the T0CKIF bit (INTSTA<6>) is set. This interrupt
can be disabled by clearing the T0CKIE control bit
(INTSTA<2>). The T0CKI interrupt can wake up the
processor from SLEEP. See Section 17.4 for details on
SLEEP operation.
6.7
The peripheral interrupt flag indicates that at least one
of the peripheral interrupts occurred (PEIF is set). The
PEIF bit is a read only bit and is a bit wise OR of all the
flag bits in the PIR registers AND’d with the correspond-
ing enable bits in the PIE registers. Some of the periph-
eral interrupts can wake the processor from SLEEP.
See Section 17.4 for details on SLEEP operation.
6.8
During an interrupt, only the returned PC value is saved
on the stack. Typically, users may wish to save key reg-
isters during an interrupt; e.g. WREG, ALUSTA and the
BSR registers. This requires implementation in software.
Example 6-2 shows the saving and restoring of infor-
mation for an Interrupt Service Routine. This is for a
simple interrupt scheme, where only one interrupt may
occur at a time (no interrupt nesting). The SFRs are
stored in the non-banked GPR area.
Example 6-2 shows the saving and restoring of infor-
mation for a more complex Interrupt Service Routine.
This is useful where nesting of interrupts is required. A
maximum of 6 levels can be done by this example. The
BSR is stored in the non-banked GPR area, while the
other registers would be stored in a particular bank.
Therefore, 6 saves may be done with this routine (since
there are 6 non-banked GPR registers). These routines
require a dedicated indirect addressing register, FSR0,
to be selected for this.
The PUSH and POP code segments could either be in
each Interrupt Service Routine, or could be subroutines
that were called. Depending on the application, other
registers may also need to be saved.
RA0/INT Interrupt
T0CKI Interrupt
Peripheral Interrupt
Context Saving During Interrupts
PIC17C7XX
DS30289B-page 39