HT36FA
pushing the program counter onto the stack and then
branching to subroutines at specified locations in the
program memory. Only the program counter is pushed
onto the stack. If the contents of the register and Status
register (STATUS) are altered by the interrupt service
program which may corrupt the desired control se-
quence, then the programmer must save the contents
first.
The internal Timer Counter 0 interrupt is initialized by
setting the Timer Counter 0 interrupt request flag (T0F;
bit 5 of INTC), caused by a Timer Counter 0 overflow.
When the interrupt is enabled, and the stack is not full
and the T0F bit is set, a subroutine call to location 08H
will occur. The related interrupt request flag (T0F) will be
reset and the EMI bit cleared to disable further inter-
rupts.
The Timer Counter 1 interrupt is operated in the same
manner as Timer Counter 0. The related interrupt con-
trol bits ET1I and T1F of the Timer Counter 1 are bit 3
and bit 6 of the INTC, respectively.
During the execution of an interrupt subroutine, other in-
terrupt acknowledgments are held until the
²RETI²
in-
struction is executed or the EMI bit and the related
interrupt control bit are set to 1 (if the stack is not full). To
return from the interrupt subroutine, the
²RET²
or
²RETI²
instruction may be invoked. RETI will set the
EMI bit to enable an interrupt service, but RET will not.
Interrupts occurring in the interval between the rising
edges of two consecutive T2 pulses, will be serviced on
the latter of the two T2 pulses, if the corresponding inter-
rupts are enabled. In the case of simultaneous requests
the priorities in the following table apply. These can be
masked by resetting the EMI bit.
Interrupt Source
Timer Counter 0 overflow
Timer Counter 1 overflow
Priority
1
2
Vector
08H
0CH
requested interrupt from being serviced. Once the inter-
rupt request flags (T0F, T1F) are set, they will remain in
the INTC register until the interrupts are serviced or
cleared by a software instruction.
It is recommended that a program does not use the
²CALL
subroutine² within the interrupt subroutine. Be-
cause interrupts often occur in an unpredictable manner
or need to be serviced immediately in some applica-
tions, if only one stack is left and enabling the interrupt is
not well controlled, once the
²CALL
subroutine² operates
in the interrupt subroutine, it may damage the original
control sequence.
Oscillator Configuration
The HT36FA provides two types of oscillator circuit for
the system clock, i.e., RC oscillator and crystal oscillator.
No matter what type of oscillator, the signal divided by 2
is used for the system clock (f
SYS
=f
OSC
/2). The HALT
mode stops the system oscillator and ignores external
signal to conserve power. If the RC oscillator is used, an
external resistor between OSC1 and VSS is required.
The system clock, divided by 4 (f
OSC2
=f
SYS
/4=f
OSC
/8), is
available on OSC2 with pull-high resistor, which can be
used to synchronize external logic. The RC oscillator
provides the most cost effective solution. However, the
frequency of the oscillation may vary with VDD, temper-
ature, 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 resonator
may be connected between OSC1 and OSC2 to replace
the crystal and to get a frequency reference, but two ex-
ternal capacitors in OSC1 and OSC2 are required.
O S C 1
V
D D
O S C 1
The Timer Counter 0/1 interrupt request flag (T0F/T1F),
Enable Timer Counter 0/1 bit (ET0I/ET1I) and Enable
Master Interrupt bit (EMI) constitute an interrupt control
register (INTC) which is located at 0BH in the data mem-
ory. EMI, ET0I, ET1I are used to control the en-
abling/disabling of interrupts. These bits prevent the
Bit No.
0
1, 4, 7
2
3
5
6
Label
EMI
¾
ET0I
ET1I
T0F
T1F
O S C 2
C r y s ta l O s c illa to r
f
O
S C
/8
R C
O S C 2
O s c illa to r
System Oscillator
Function
Controls the Master (Global) interrupt (1=enabled; 0=disabled)
Unused bit, read as
²0²
Controls the Timer Counter 0 interrupt (1=enabled; 0=disabled)
Controls the Timer Counter 1 interrupt (1=enabled; 0=disabled)
Internal Timer Counter 0 request flag (1=active; 0=inactive)
Internal Timer Counter 1 request flag (1=active; 0=inactive)
INTC (0BH) Register
Rev. 1.20
9
February 26, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
