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PIC18F66K80 FAMILY

DS39977F-page 106

2010-2012 Microchip Technology Inc.

6.2

PIC18 Instruction Cycle

6.2.1

CLOCKING SCHEME

The microcontroller clock input, whether from an

internal or external source, is internally divided by four

to generate four non-overlapping quadrature clocks

(Q1, Q2, Q3 and Q4). Internally, the Program Counter

is incremented on every Q1, with the instruction

fetched from the program memory and latched into the

Instruction Register (IR) during Q4.

The instruction is decoded and executed during the

following Q1 through Q4. The clocks and instruction

execution flow are shown in Figure 6-4.

6.2.2

INSTRUCTION FLOW/PIPELINING

An “Instruction Cycle” consists of four Q cycles, Q1

through Q4. The instruction fetch and execute are pipe-

lined in such a manner that a fetch takes one instruction

cycle, while the decode and execute take another

instruction cycle. However, due to the pipelining, each

instruction effectively executes in one cycle. If an

instruction (such as GOTO) causes the Program Coun-

ter to change, two cycles are required to complete the

instruction. (See Example 6-3.)

A fetch cycle begins with the Program Counter (PC)

incrementing in Q1.

In the execution cycle, the fetched instruction is latched

into the Instruction Register (IR) in cycle, Q1. This

instruction is then decoded and executed during the

Q2, Q3 and Q4 cycles. Data memory is read during Q2

(operand read) and written during Q4 (destination

write).

FIGURE 6-4:

CLOCK/INSTRUCTION CYCLE

EXAMPLE 6-3:

INSTRUCTION PIPELINE FLOW

OSC1

OSC2/CLKO

(RC mode)

PC + 2

PC + 4

Fetch INST (PC)

Execute INST (PC – 2)

Fetch INST (PC + 2)

Execute INST (PC)

Fetch INST (PC + 4)

Execute INST (PC + 2)

Internal

Phase

Clock

All instructions are single cycle, except for any program branches. These take two cycles since the fetch instruction

is “flushed” from the pipeline while the new instruction is being fetched and then executed.

TCY0TCY1TCY2TCY3TCY4TCY5

1. MOVLW 55h

Fetch 1

Execute 1

2. MOVWF PORTB

Fetch 2

Execute 2

3. BRA

SUB_1

Fetch 3

Execute 3

4. BSF

PORTA, BIT3 (Forced NOP)

Fetch 4

Flush (NOP)

5. Instruction @ address SUB_1

Fetch SUB_1 Execute SUB_1

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