COP888EB datasheet(39/75 Pages) NSC

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Interrupts (Continued)

MASKABLE INTERRUPTS

All interrupts other than the Software Trap are maskable.

Each maskable interrupt has an associated enable bit and

pending flag bit. The pending bit is set to 1 when the interrupt

condition occurs. The state of the interrupt enable bit, com-

bined with the GIE bit determines whether an active pending

flag actually triggers an interrupt. All of the maskable inter-

rupt pending and enable bits are contained in mapped con-

trol registers, and thus can be controlled by the software.

A maskable interrupt condition triggers an interrupt under the

following conditions:

The enable bit associated with that interrupt is set.

The GIE bit is set.

The device is not processing a non-maskable interrupt.

(If a non-maskable interrupt is being serviced, a

maskable interrupt must wait until that service routine is

completed.)

An interrupt is triggered only when all of these conditions are

met at the beginning of an instruction. If different maskable

interrupts meet these conditions simultaneously, the highest

priority interrupt will be serviced first, and the other pending

interrupts must wait.

Upon Reset, all pending bits, individual enable bits, and the

GIE bit are reset to zero. Thus, a maskable interrupt condi-

tion cannot trigger an interrupt until the program enables it by

setting both the GIE bit and the individual enable bit. When

enabling an interrupt, the user should consider whether or

not a previously activated (set) pending bit should be ac-

knowledged. If, at the time an interrupt is enabled, any pre-

vious occurrences of the interrupt should be ignored, the as-

sociated pending bit must be reset to zero prior to enabling

the interrupt. Otherwise, the interrupt may be simply en-

abled; if the pending bit is already set, it will immediately trig-

ger an interrupt. A maskable interrupt is active if its associ-

ated enable and pending bits are set.

An interrupt is an asychronous event which may occur be-

fore, during, or after an instruction cycle. Any interrupt which

occurs during the execution of an instruction is not acknowl-

edged until the start of the next normally executed instruction

is to be skipped, the skip is performed before the pending in-

terrupt is acknowledged.

At the start of interrupt acknowledgment, the following ac-

tions occur:

The GIE bit is automatically reset to zero, preventing any

subsequent maskable interrupt from interrupting the cur-

rent service routine. This feature prevents one maskable

interrupt from interrupting another one being serviced.

The address of the instruction about to be executed is

pushed onto the stack.

The program counter (PC) is loaded with 00FF Hex,

causing a jump to that program memory location.

The device requires seven instruction cycles to perform the

actions listed above.

If the user wishes to allow nested interrupts, the interrupts

service routine may set the GIE bit to 1 by writing to the PSW

the current service routine. If nested interrupts are allowed,

caution must be exercised. The user must write the program

in such a way as to prevent stack overflow, loss of saved

context information, and other unwanted conditions.

The interrupt service routine stored at location 00FF Hex

should use the VIS instruction to determine the cause of the

interrupt, and jump to the interrupt handling routine corre-

sponding to the highest priority enabled and active interrupt.

Alternately, the user may choose to poll all interrupt pending

and enable bits to determine the source(s) of the interrupt. If

more than one interrupt is active, the user’s program must

decide which interrupt to service.

Within a specific interrupt service routine, the associated

pending bit should be cleared. This is typically done as early

as possible in the service routine in order to avoid missing

the next occurrence of the same type of interrupt event.

Thus, if the same event occurs a second time, even while the

first occurrence is still being serviced, the second occur-

rence will be serviced immediately upon return from the cur-

rent interrupt routine.

An interrupt service routine typically ends with an RETI in-

struction. This instruction sets the GIE bit back to 1, pops the

address stored on the stack, and restores that address to the

program counter. Program execution then proceeds with the

next instruction that would have been executed had there

been no interrupt. If there are any valid interrupts pending,

the highest-priority interrupt is serviced immediately upon re-

turn from the previous interrupt.

VIS INSTRUCTION

The general interrupt service routine, which starts at address

00FF Hex, must be capable of handling all types of inter-

rupts. The VIS instruction, together with an interrupt vector

table, directs the device to the specific interrupt handling rou-

tine based on the cause of the interrupt.

VIS is a single-byte instruction, typically used at the very be-

ginning of the general interrupt service routine at address

00FF Hex, or shortly after that point, just after the code used

for context switching. The VIS instruction determines which

enabled and pending interrupt has the highest priority, and

causes an indirect jump to the address corresponding to that

interrupt source. The jump addresses (vectors) for all pos-

sible interrupts sources are stored in a vector table.

The vector table may be as long as 32 bytes (maximum of 16

vectors) and resides at the top of the 256-byte block contain-

ing the VIS instruction. However, if the VIS instruction is at

the very top of a 256-byte block (such as at 00FF Hex), the

vector table resides at the top of the next 256-byte block.

Thus, if the VIS instruction is located somewhere between

00FF and 01DF Hex (the usual case), the vector table is lo-

cated between addresses 01E0 and 01FF Hex. If the VIS in-

struction is located between 01FF and 02DF Hex, then the

vector table is located between addresses 02E0 and 02FF

Hex, and so on.

Each vector is 15 bits long and points to the beginning of a

specific interrupt service routine somewhere in the 32 kbyte

memory space. Each vector occupies two bytes of the vector

table, with the higher-order byte at the lower address. The

vectors are arranged in order of interrupt priority. The vector

of the maskable interrupt with the lowest rank is located to

0yE0 (higher-order byte) and 0yE1 (lower-order byte). The

next priority interrupt is located at 0yE2 and 0yE3, and so

forth in increasing rank. The Software Trap has the highest

rank and its vector is always located at 0yFE and 0yFF. The

number of interrupts which can become active defines the

size of the table.

Table 10 shows the types of interrupts, the interrupt arbitra-

tion ranking, and the locations of the corresponding vectors

in the vector table.

The vector table should be filled by the user with the memory

locations of the specific interrupt service routines. For ex-

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