HT66F20 View Datasheet(PDF) - Holtek Semiconductor

Part Name

Description

Manufacturer

HT66F20

Enhanced A/D Flash Type MCU 8-Bit MCU with EEPROM

Holtek Semiconductor 

HT66F20/HT66F30/HT66F40/HT66F50/HT66F60

HT66FU30/HT66FU40/HT66FU50/HT66FU60

Program Counter

During program execution, the Program Counter is used

to keep track of the address of the next instruction to be

executed. It is automatically incremented by one each

time an instruction is executed except for instructions,

such as ²JMP² or ²CALL² that demand a jump to a

non-consecutive Program Memory address. Only the

lower 8 bits, known as the Program Counter Low Regis-

ter, are directly addressable by the application program.

When executing instructions requiring jumps to

non-consecutive addresses such as a jump instruction,

a subroutine call, interrupt or reset, etc., the

microcontroller manages program control by loading the

required address into the Program Counter. For condi-

tional skip instructions, once the condition has been

met, the next instruction, which has already been

fetched during the present instruction execution, is dis-

carded and a dummy cycle takes its place while the cor-

rect instruction is obtained.

Device

HT66F20

HT66F30

HT66F40

HT66F50

HT66F60

Program Counter

Program Counter

High Byte

PCL Register

PC9, PC8

PC10~PC8

PC11~PC8

PCL7~PCL0

PC12~PC8

PC13~PC8

Program Counter

The lower byte of the Program Counter, known as the

Program Counter Low register or PCL, is available for

program control and is a readable and writeable register.

By transferring data directly into this register, a short pro-

gram jump can be executed directly, however, as only

this low byte is available for manipulation, the jumps are

limited to the present page of memory, that is 256 loca-

tions. When such program jumps are executed it should

also be noted that a dummy cycle will be inserted. Manip-

ulating the PCL register may cause program branching,

so an extra cycle is needed to pre-fetch.

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, and is nei-

ther readable nor writeable. At a subroutine call or

interrupt acknowledge signal, the contents of the Pro-

gram Counter are pushed onto the stack. At the end of a

subroutine or an interrupt routine, signaled by a return

instruction, RET or RETI, the Program Counter is re-

stored 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.

If the stack is overflow, the first Program Counter save in

the stack will be lost.

P ro g ra m C o u n te r

T o p o f S ta c k

S ta c k

P o in te r

S ta c k L e v e l 1

S ta c k L e v e l 2

S ta c k L e v e l 3

B o tto m o f S ta c k

S ta c k L e v e l N

P ro g ra m

M e m o ry

Device

HT66F20/HT66F30

HT66F40/HT66F50

HT66F60

Stack Levels

4

8

12

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

Rev. 1.10

20

February 1, 2010

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