MC68HC05P4A Datasheet, PDF (21/72 Pages) Advanced Analogic Technologies – Microcontrollers

Chapter 3
Central Processor Unit (CPU)
3.1 Introduction
This section describes the five CPU registers. CPU registers are not part of the memory map.
3.2 Accumulator (A)
The accumulator is a general-purpose 8-bit register used to hold operands and results of arithmetic
calculations or data manipulations.
7
0
A
3.3 Index Register (X)
The index register is an 8-bit register used for the indexed addressing value to create an effective
address. The index register also may be used as a temporary storage area.
7
0
X
3.4 Condition Code Register (CCR)
The CCR is a 5-bit register in which four bits are used to indicate the results of the instruction just
executed, and the fifth bit indicates whether interrupts are masked. These bits can be tested individually
by a program, and specific actions can be taken as a result of their state. Each bit is explained in the
following paragraphs.
CCR
H I NZC
3.4.1 H — Half Carry
This bit is set during ADD and ADC operations to indicate that a carry occurred between bits 3 and 4.
3.4.2 I — Interrupt
When this bit is set, timer and external interrupts are masked (disabled). If an interrupt occurs while this
bit is set, the interrupt is latched and processed as soon as the interrupt bit is cleared.
3.4.3 N — Negative
When set, this bit indicates that the result of the last arithmetic, logical, or data manipulation was negative.
MC68HC05P4A Data Sheet, Rev. 7.1
Freescale Semiconductor
21