HT82B40R datasheet(21/57 Pages) HOLTEK

21 / 57 page

HT82B40R/HT82B40A

Rev. 1.10

September 4, 2009

org 04h

; USB interrupt vector

reti

org 08h

; Timer/Event Counter interrupt vector

jmp tmr0int

; jump here when Timer0 overflows

org 20h

; main program

;internal Timer/Event Counter 0 interrupt routine

Tmr0int:

; Timer/Event Counter 0 main program placed here

reti

begin:

;setup Timer registers

mov a,09bh

; setup Timer preload value

mov tmr0,a;

mov a,080h

; setup Timer control register

mov tmr0c,a

; timer mode

; setup interrupt register

mov a,005h

; enable master interrupt and timer interrupt

mov intc,a

set tmr0c.4

; start Timer/Event Counter - note mode bits must be previously setup

Interrupts

Interrupts are an important part of any microcontroller

system. When an internal function such as a Timer/Event

Counter overflow or a USB interrupt occur, their corre-

sponding interrupt will enforce a temporary suspension of

the main program allowing the microcontroller to direct

attention to their respective needs. while the internal in-

terrupts are controlled by the Timer/Event Counter over-

flow, USB interrupt or reception.

Interrupt Register

Overall interrupt control, which means interrupt enabling

and request flag setting, is controlled by an interrupt

control register. By controlling the appropriate enable

bits in this register each individual interrupt can be en-

abled or disabled. Also when an interrupt occurs, the

corresponding request flag will be set by the

microcontroller. The global enable flag if cleared to zero

will disable all interrupts.

Interrupt Operation

When a USB interrupt occurs or one of the Timer/Event

Counters overflow, if their appropriate interrupt enable

bit is set, the Program Counter, which stores the ad-

dress of the next instruction to be executed, will be

transferred onto the stack. The Program Counter will

then be loaded with a new address which will be the

value of the corresponding interrupt vector. The

microcontroller will then fetch its next instruction from

this interrupt vector. The instruction at this vector will

usually be a JMP statement which will jump to another

section of program which is known as the interrupt ser-

vice routine. Here is located the code to control the ap-

propriate interrupt. The interrupt service routine must be

terminated with a RETI statement, which retrieves the

original Program Counter address from the stack and al-

lows the microcontroller to continue with normal execu-

tion at the point where the interrupt occurred.

The various interrupt enable bits, together with their as-

sociated request flags, are shown in the accompanying

diagram with their order of priority.

Once an interrupt subroutine is serviced, all the other in-

terrupts will be blocked, as the EMI bit will be cleared au-

tomatically. This will prevent any further interrupt nesting

from occurring. However, if other interrupt requests oc-

cur during this interval, although the interrupt will not be

immediately serviced, the request flag will still be re-

corded. If an interrupt requires immediate servicing

while the program is already in another interrupt service

routine, the EMI bit should be set after entering the rou-

tine, to allow interrupt nesting. If the stack is full, the in-

terrupt request will not be acknowledged, even if the

related interrupt is enabled, until the Stack Pointer is

decremented. If immediate service is desired, the stack

must be prevented from becoming full.

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