NEC PD17062 Manuale Utente

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The timer interrupt function issues an interrupt request at the negative-going edge of the timer interrupt

pulse specified in the timer mode select register.

The timer interrupt request corresponds to the IRQBTM0 flag in the interrupt request register on a one-to-

one basis.  When an interrupt is requested, the corresponding IRQBTM0 flag is set to 1.

In other words, when a timer interrupt request pulse falls, the IRQBTM0 flag is set to 1.

As described in Chapter 11, to use the timer interrupt function, it is necessary not only to issue an interrupt

request but also to execute the EI instruction, which enables all interrupts, and enable the timer interrupt.

The timer interrupt is enabled by setting the IPBTM0 flag to 1 in the interrupt permission register.

To put in another way, if the EI instruction has been executed, and the IPBTM0 flag is set to 1, an interrupt

request is accepted when the IRQBTM0 flag is set to 1.

When a timer interrupt request is accepted, program control is passed to program memory address 0003H.

When the interrupt request is accepted, the IRQBTM0 flag is reset to 0.

Fig. 12-8 shows the relationship between the timer interrupt pulse and the IRQBTM0 flag.

#

IRQBTM0

IPBTM0

INTE

FF    

EI

DI

Timer interrupt pulse

The negative-going edge 
of the timer interrupt pulse 
sets the IRQBTM0 flag.

The EI instruction is executed, 
but the interrupt request is 
not accepted because the 
IPBTM0 flag is not set.

The timer interrupt request is 
accepted at the same time 
the IPBTM0 flag is set.

Timer interrupt
request accepted

Interrupt pending

Interrupt enabled

At this point, note the following:  Once the IRQBTM0 flag is set when a timer interrupt is disabled by the

DI instruction or the IPBTM0 flag, the corresponding interrupt request is accepted immediately when the EI

instruction is executed or the IPBTM0 flag is set.

In the above case, writing 0 to the IRQBTM0 flag can cancel the interrupt request.

Meanwhile, writing 1 to the IRQBTM0 flag amounts to issuing an interrupt request.

Accepting a timer interrupt request uses one level of stack.

When an interrupt request is accepted, the contents of the bank register and index enable flag are saved

automatically.

A RETI instruction is used to return from an interrupt handling routine.  This instruction is dedicated to use

for this purpose.

See Chapters 4 and 11 for details.

Sections  12.5.1 and 12.5.2 describe an example of using a timer interrupt and a timer interrupt error,

respectively.

See Chapter 11 for relationships with other types of interrupts (INT

NC

 pin, V

SYNC

 pin, and serial interface).