To ensure reliable operation, the user should always use the
VIS instruction to determine the source of an interrupt. Al-
though it is possible to poll the pending bits to detect the
source of an interrupt, this practice is not recommended. The
use of polling allows the standard arbitration ranking to be
altered, but the reliability of the interrupt system is compro-
mised. The polling routine must individually test the enable
and pending bits of each maskable interrupt. If a Software
Trap interrupt should occur, it will be serviced last, even
though it should have the highest priority. Under certain
conditions, a Software Trap could be triggered but not ser-
viced, resulting in an inadvertent “locking out” of all
maskable interrupts by the Software Trap pending flag.
Problems such as this can be avoided by using VIS
instruction.
18.0 Interrupts (Continued)
The default VIS interrupt vector can be useful for applica-
tions in which time critical interrupts can occur during the
servicing of another interrupt. Rather than restoring the pro-
gram context (A, B, X, etc.) and executing the RETI instruc-
tion, an interrupt service routine can be terminated by return-
ing to the VIS instruction. In this case, interrupts will be
serviced in turn until no further interrupts are pending and
the default VIS routine is started. After testing the GIE bit to
ensure that execution is not erroneous, the routine should
restore the program context and execute the RETI to return
to the interrupted program.
This technique can save up to fifty instruction cycles (tC), or
more, (25 µs at 10 MHz oscillator) of latency for pending
interrupts with a penalty of fewer than ten instruction cycles
if no further interrupts are pending.
TABLE 33. Interrupt Vector Table
Source Description
Vector Address (Note 16)
Arbitration Ranking
(Hi-Low Byte)
(1) Highest
(2)
Software
INTR Instruction
0yFE–0yFF
Reserved for NMI
External
0yFC–0yFD
0yFA–0yFB
0yF8–0yF9
0yF6–0yF7
0yF4–0yF5
0yF2–0yF3
0yF0–0yF1
0yEE–0yEF
0yEC–0yED
0yEA–0yEB
0yE8–0yE9
0yE6–0yE7
0yE4–0yE5
0yE2–0yE3
0yE0–0yE1
(3)
G0
(4)
Timer T0
Underflow
T1A/Underflow
T1B
(5)
Timer T1
(6)
Timer T1
(7)
MICROWIRE/PLUS
Reserved
USART
BUSY Low
(8)
(9)
Receive
(10)
(11)
USART
Transmit
Timer T2
T2A/Underflow
T2B
(12)
(13)
(14)
(15)
(16) Lowest
Timer T2
Timer T3
T2A/Underflow
T3B
Timer T3
Port L/Wakeup
Default VIS
Port L Edge
Reserved
Note 16: y is a variable which represents the VIS block. VIS and the vector table must be located in the same 256-byte block except if VIS is located at the last
address of a block. In this case, the table must be in the next block.
18.3.1 VIS Execution
the active interrupt with the highest arbitration ranking. This
vector is read from program memory and placed into the PC
which is now pointed to the 1st instruction of the service
routine of the active interrupt with the highest arbitration
ranking.
When the VIS instruction is executed it activates the arbitra-
tion logic. The arbitration logic generates an even number
between E0 and FE (E0, E2, E4, E6 etc....) depending on
which active interrupt has the highest arbitration ranking at
the time of the 1st cycle of VIS is executed. For example, if
the software trap interrupt is active, FE is generated. If the
external interrupt is active and the software trap interrupt is
not, then FA is generated and so forth. If no active interrupt
is pending, than E0 is generated. This number replaces the
lower byte of the PC. The upper byte of the PC remains
unchanged. The new PC is therefore pointing to the vector of
Figure 32 illustrates the different steps performed by the VIS
instruction. Figure 33 shows a flowchart for the VIS instruc-
tion.
The non-maskable interrupt pending flag is cleared by the
RPND (Reset Non-Maskable Pending Bit) instruction (under
certain conditions) and upon RESET.
59
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