Rainbow Electronics HT49R70A-1 - Manual (Page 12)

HT49R70A-1
Rev. 1.00 12 December 4, 2001
The Timer/Event Counter 0 interrupt request flag (T0F),
external interrupt 1 request flag (EIF1), external inter-
rupt 0 request flag (EIF0), enable Timer/Event Counter
0 interrupt bit (ET0I), enable external interrupt 1 bit
(EEI1), enable external interrupt 0 bit (EEI0), and en-
able master interrupt bit (EMI) make up of the Interrupt
Control register 0 (INTC0) which is located at 0BH in the
RAM. The real time clock interrupt request flag (RTF),
time base interrupt request flag (TBF), Timer/Event
Counter 1 interrupt request flag (T1F), enable real time
clock interrupt bit (ERTI), and enable time base interrupt
bit (ETBI), enable Timer/Event Counter 1 interrupt bit
(ET1I) on the other hand, constitute the Interrupt Control
register 1 (INTC1) which is located at 1EH in the RAM.
EMI, EEI0, EEI1, ET0I, ET1I, ETBI, and ERTI are all
used to control the enable/disable status of interrupts.
These bits prevent the requested interrupt from being
serviced. Once the interrupt request flags (RTF, TBF, T0F,
T1F, EIF1, EIF0) are all set, they remain in the INTC1 or
INTC0 respectively until the interrupts are serviced or
cleared by a software instruction.
It is recommended that a program should not use the
²CALL subroutine² within the interrupt subroutine. It¢s be-
cause interrupts often occur in an unpredictable manner
or require to be serviced immediately in some applica-
tions. During that period, if only one stack is left, and en-
abling the interrupt is not well controlled, operation of the
²call² in the interrupt subroutine may damage the origi-
nal control sequence.
Oscillator configuration
The HT49R70A-1 provides three oscillator circuits for
system clocks, i.e., RC oscillator, crystal oscillator and
32768Hz crystal oscillator, determined by options. No
matter what type of oscillator is selected, the signal is
used for the system clock. The HALT mode stops the
system oscillator (RC and crystal oscillator only) and ig-
nores external signal in order to conserve power. The
32768Hz crystal oscillator (system oscillator) still runs at
HALT mode. If the 32768Hz crystal oscillator is selected
as the system oscillator, the system oscillator is not
stopped; but the instruction execution is stopped. Since
the system oscillator or oscillator) is also designed for
timing purposes, the internal timing (RTC, time base,
WDT) operation still runs even if the system enters the
HALT mode.
Of the three oscillators, if the RC oscillator is used, an
external resistor between OSC1 and VSS is required,
and the range of the resistance should be from 40k W to
680k W. The system clock, divided by 4, is available on
OSC2 with pull-high resistor, which can be used to syn-
chronize external logic. The RC oscillator provides the
most cost effective solution. However, the frequency of
the oscillation may vary with VDD, temperature, and the
chip itself due to process variations. It is therefore, not
suitable for timing sensitive operations where accurate
oscillator frequency is desired.
On the other hand, if the crystal oscillator is selected, a
crystal across OSC1 and OSC2 is needed to provide
the feedback and phase shift required for the oscillator,
and no other external components are required. A reso-
nator may be connected between OSC1 and OSC2 to
replace the crystal and to get a frequency reference, but
two external capacitors in OSC1 and OSC2 are re-
quired.
There is another oscillator circuit designed for the real
time clock. In this case, only the 32.768kHz crystal oscil-
lator can be applied. The crystal should be connected
between OSC3 and OSC4.
The RTC oscillator circuit can be controlled to oscillate
quickly by setting the ²QOSC² bit (bit 4 of RTCC). It is
recommended to turn on the quick oscillating function
upon power on, and then turn it off after 2 seconds.
The WDT oscillator is a free running on-chip RC oscilla-
tor, and no external components are required. Although
the system enters the power down mode, the system
clock stops, and the WDT oscillator still works with a pe-
riod of approximately 78 ms. The WDT oscillator can be
disabled by options to conserve power.
Watchdog Timer - WDT
The WDT clock source is implemented by a dedicated
RC oscillator (WDT oscillator) or an instruction clock
(system clock/4) or a real time clock oscillator (RTC os-
cillator). The timer is designed to prevent a software
malfunction or sequence from jumping to an unknown
location with unpredictable results. The WDT can be
disabled by options. But if the WDT is disabled, all exe-
cutions related to the WDT lead to no operation.
C r y s t a l O s c i l l a t o r R C O s c i l l a t o r
O S C 1
O S C 1
O S C 2 f S Y S / 4 O S C 2
V
D D
3 2 7 6 8 H z C r y s t a l / R T C O s c i l l a t o r
O S C 3
O S C 4
System oscillator