HT56RB688 datasheet(23/164 Pages) HOLTEK

23 / 164 page

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 writable register. By transferring data directly into

this register, a short program 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 locations. When such

program jumps are executed it should also be noted that a dummy cycle will be inserted.

The lower byte of the Program Counter is fully accessible under program control. Manipulating the

PCL might cause program branching, so an extra cycle is needed to pre-fetch. Further information on

the PCL register can be found in the Special Function Register section.

Stack

This is a special part of the memory which is used to save the contents of the Program Counter only.

The stack is neither part of the data nor part of the program space, and is neither readable nor writeable.

The activated level is indexed by the Stack Pointer, SP, and is neither readable nor writeable. At a

subroutine call or interrupt acknowledge signal, the contents of the Program 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 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 acknowledge signal will be inhibited. When the Stack Pointer is decremented, by RET or RETI, the

interrupt will be serviced. This feature prevents stack overflow allowing the programmer to use the

structure more easily. However, when the stack is full, a CALL subroutine instruction 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 microcontrollers that carries out arithmetic

and logic operations of the instruction set. Connected to the main microcontrollers data bus, the ALU

receives related instruction 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 operations may

result in carry, borrow or other status changes, the status register will be correspondingly updated 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

HT56RB688

TinyPowerTM A/D type Smart Card OTP MCU

with LCD, DAC, ISO 7816 and USB Interfaces

Rev. 1.00

December 2, 2011

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 1 2

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

HT56RB688 Datasheet(PDF) 23 Page - Holtek Semiconductor Inc

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