HT45R37V datasheet(13/90 Pages) HOLTEK

13 / 90 page

HT45R37V

Rev. 1.00

October 20, 2009

Stack

This is a special part of the memory which is used to

save the contents of the Program Counter only. The

stack has multiple levels depending upon the device

and is neither part of the data nor part of the program

space, and is neither readable nor writeable. The acti-

vated level is indexed by the Stack Pointer, SP, and is

neither readable nor writeable. At a subroutine call or in-

terrupt acknowledge signal, the contents of the Program

Counter are pushed onto the stack. At the end of a sub-

routine or an interrupt routine, signaled by a return in-

struction, RET or RETI, the Program Counter is restored

to its previous value from the stack. After a device reset,

the Stack Pointer will point to the top of the stack.

If the stack is full and an enabled interrupt takes place,

the interrupt request flag will be recorded but the ac-

knowledge signal will be inhibited. When the Stack

Pointer is decremented, by RET or RETI, the interrupt

will be serviced. This feature prevents stack overflow al-

lowing the programmer to use the structure more easily.

However, when the stack is full, a CALL subroutine in-

struction can still be executed which will result in a stack

overflow. Precautions should be taken to avoid such

cases which might cause unpredictable program

branching.

Arithmetic and Logic Unit

- ALU

The arithmetic-logic unit or ALU is a critical area of the

microcontroller that carries out arithmetic and logic oper-

ations of the instruction set. Connected to the main

microcontroller data bus, the ALU receives related in-

struction codes and performs the required arithmetic or

logical operations after which the result will be placed in

the specified register. As these ALU calculation or oper-

ations may result in carry, borrow or other status

changes, the status register will be correspondingly up-

dated to reflect these changes. The ALU supports the

following functions:

· Arithmetic operations: ADD, ADDM, ADC, ADCM,

SUB, SUBM, SBC, SBCM, DAA

· Logic operations: AND, OR, XOR, ANDM, ORM,

XORM, CPL, CPLA

· Rotation RRA, RR, RRCA, RRC, RLA, RL, RLCA,

RLC

· Increment and Decrement INCA, INC, DECA, DEC

· Branch decision, JMP, SZ, SZA, SNZ, SIZ, SDZ,

SIZA, SDZA, CALL, RET, RETI

Program Memory

The Program Memory is the location where the user

code or program is stored. For these device the Pro-

gram Memory is an OTP type, which means it can be

programmed only one time. By using the appropriate

programming tools, this OTP memory device offer users

the flexibility to conveniently debug and develop their

applications while also offering a means of field pro-

gramming.

Structure

The Program Memory has a capacity of 4K by 15 bits.

The Program Memory is addressed by the Program

Counter and also contains data, table information and

interrupt entries. Table data, which can be setup in any

location within the Program Memory, is addressed by a

separate table pointer register.

Special Vectors

Within the Program Memory, certain locations are re-

served for special usage such as reset and interrupts.

· Location 000H

This vector is reserved for use by the device reset for

program initialisation. After a device reset is initiated, the

program will jump to this location and begin execution.

· Location 004H

This vector is used by the external interrupt 0. If the

external interrupt pin receives an active edge, the pro-

gram will jump to this location and begin execution if

the external interrupt is enabled and the stack is not

full.

· Location 008H

This vector is used by the external interrupt 1. If the

external interrupt pin receives an active edge, the pro-

gram will jump to this location and begin execution if

the external interrupt is enabled and the stack is not

full.

· Location 00CH

This internal vector is used by the Timer/Event Coun-

ter 0. If a Timer/Event Counter 0 overflow occurs, the

program will jump to this location and begin execution

if the timer/event counter interrupt is enabled and the

stack is not full.

P r o g r a m C o u n t e r

S t a c k L e v e l 1

S t a c k L e v e l 2

S t a c k L e v e l 3

S t a c k L e v e l 6

P r o g r a m

M e m o r y

T o p o f S t a c k

S t a c k

P o i n t e r

B o t t o m o f S t a c k

HT45R37V Datasheet(PDF) 13 Page - Holtek Semiconductor Inc

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