B3-17

EM78P156E

008H. Once in the interrupt service routine, the source of an interrupt can be determined by polling the flag bits

in RF. The interrupt flag bit must be cleared by instructions before leaving the interrupt service routine and

enabling interrupts to avoid recursive interrupts.

The flag (except ICIF bit) in the Interrupt Status Register (RF) is set regardless of the status of its mask bit or the

execution of ENI. Note that the outcome of RF will be the logic AND of RF and IOCF. Refer to Fig.10. The

RETI instruction ends the interrupt routine and enables the global interrupt (the execution of ENI).

When an interrupt is generated by the INT instruction (when enabled), the next instruction will be fetched from

address 001H

Q

Q

P

R

CLK

C

L

D

Q

Q

P

R

CLK

C

L

D

/INT

IOD

VCC

/RESET

RF

IRQn

.

.

IRQm

ENI/DISI

RPRD

IOCFWR

IOCF

IOCFRD

RFWR

/IRQn

VI.7 Oscillator

1.

Oscillator Modes

EM78P156E can be operated in four different oscillator modes. There are Internal Capacitor oscillator mode (IRC),

External RC oscillator mode (ERC), High XTAL oscillator mode (HXT) and Low XTAL oscillator mode (LXT). Users

can select one of them by programming MS and HLF in the CODE option register. Table 6 depicts how these four modes

to be defined.

The up-limited operation frequency of crystal/resonator on the different VDDs is listed in Table 7.

Table 6 Oscillator Modes defined by MS, HLF, HLP and IRCEN

Mode

MS

HLF

HLP

IRCEN

External RC oscillator mode

0

*X

*X

1

High XTAL oscillator mode

1

1

*X

*X

Low XTAL oscillator mode

1

0

0

*X

Internal C, External R oscillator mode

0

*X

*X

0

<Note> 1. X, Do not care

2. The transient point of system frequency between HXT and LXY is around 400 KHz.

Fig. 10 Interrupt input circuit