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10.2. Interrupt Priorities
Each interrupt source can be individually programmed to one of two priority levels: low or high. A low prior-
ity interrupt service routine can be preempted by a high priority interrupt. A high priority interrupt cannot be
preempted. Each interrupt has an associated interrupt priority bit in an SFR (IP, EIP1, or EIP2) used to
configure its priority level. Low priority is the default. If two interrupts are recognized simultaneously, the
interrupt with the higher priority is serviced first. If both interrupts have the same priority level, a fixed prior-
ity order is used to arbitrate, given in Table 10.1.
10.3. Interrupt Latency
Interrupt response time depends on the state of the CPU when the interrupt occurs. Pending interrupts are
sampled and priority decoded each system clock cycle. Therefore, the fastest possible response time is
5 system clock cycles: 1 clock cycle to detect the interrupt and 4 clock cycles to complete the LCALL to the
ISR. Additional clock cycles will be required if a cache miss occurs (see Section 14 for more details). If an
interrupt is pending when a RETI is executed, a single instruction is executed before an LCALL is made to
service the pending interrupt. Therefore, the maximum response time for an interrupt (when no other inter-
rupt is currently being serviced or the new interrupt is of greater priority) is when the CPU is performing an
RETI instruction followed by a DIV as the next instruction, and a cache miss event also occurs. If the CPU
is executing an ISR for an interrupt with equal or higher priority, the new interrupt will not be serviced until
the current ISR completes, including the RETI and following instruction.
Table 10.1. Interrupt Summary
Interrupt Priority
Enable
Flag
Priority
Control
Interrupt Source
Pending Flag
Vector
Order
Always
Enabled
Always
Highest
Reset
0x0000
Top None
N/A N/A
External Interrupt 0 (/INT0) 0x0003
Timer 0 Overflow 0x000B
External Interrupt 1 (/INT1) 0x0013
0
1
2
3
IE0 (TCON.1)
TF0 (TCON.5)
IE1 (TCON.3)
TF1 (TCON.7)
Y
Y
Y
Y
Y
Y
Y
Y
EX0 (IE.0) PX0 (IP.0)
ET0 (IE.1) PT0 (IP.1)
EX1 (IE.2) PX1 (IP.2)
ET1 (IE.3) PT1 (IP.3)
Timer 1 Overflow
0x001B
RI0 (SCON0.0)
TI0 (SCON0.1)
UART0
0x0023
4
5
Y
Y
N
N
ES0 (IE.4) PS0 (IP.4)
ET2 (IE.5) PT2 (IP.5)
TF2H (TMR2CN.7)
TF2L (TMR2CN.6)
SPIF (SPI0CN.7)
WCOL (SPI0CN.6)
MODF (SPI0CN.5)
RXOVRN (SPI0CN.4)
Timer 2 Overflow
SPI0
0x002B
0x0033
ESPI0
(IE.6)
PSPI0
(IP.6)
6
Y
N
ESMB0
(EIE1.0)
N/A
PSMB0
(EIP1.0)
N/A
SMB0
0x003B
0x0043
0x004B
7
8
9
SI (SMB0CN.0)
Y
N
RESERVED
ADC0 Window
Comparator
N/A
AD0WINT
(ADC0CN.5)
N/A N/A
EWADC0 PWADC0
Y
Y
N
N
(EIE1.2)
EADC0
(EIE1.3)
(EIP1.2)
PADC0
(EIP1.3)
ADC0 End of Conversion
0x0053
10
AD0INT (ADC0STA.5)
108
Rev. 1.0
SILICON [ SILICON ]
