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PIC16C63A/65B/73B/74B

DS30605C-page 14

2000 Microchip Technology Inc.

3.1

Clocking Scheme/Instruction

Cycle

The clock input (from OSC1) is internally divided by

four to generate four non-overlapping quadrature

clocks, namely Q1, Q2, Q3 and Q4. Internally, the pro-

gram counter (PC) is incremented every Q1, the

instruction is fetched from the program memory and

latched into the instruction register in Q4. The instruc-

tion is decoded and executed during the following Q1

through Q4. The clocks and instruction execution flow

is shown in Figure 3-2.

3.2

Instruction Flow/Pipelining

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

Q2, Q3 and Q4). The instruction fetch and execute are

pipelined such that fetch takes one instruction cycle,

while decode and execute takes another instruction

cycle. However, due to the pipelining, each instruction

effectively executes in one cycle. If an instruction

causes the program counter to change (e.g., GOTO),

then two cycles are required to complete the instruction

(Example 3-1).

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 3-2:

CLOCK/INSTRUCTION CYCLE

EXAMPLE 3-1:

INSTRUCTION PIPELINE FLOW

OSC1

OSC2/CLKOUT

(RC mode)

PC+1

PC+2

Fetch INST (PC)

Execute INST (PC-1)

Fetch INST (PC+1)

Execute INST (PC)

Fetch INST (PC+2)

Execute INST (PC+1)

Internal

phase

clock

Note:

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. CALL

SUB_1

Fetch 3

Execute 3

4. BSF

PORTA, BIT3 (Forced NOP)

Fetch 4

Flush

5. Instruction @ address SUB_1

Fetch SUB_1 Execute SUB_1

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