PIC12CE518 datasheet(10/88 Pages) MICROCHIP

10 / 88 page

PIC12CE5XX

DS40172A-page 10

Preliminary

3.1

Clocking Scheme/Instruction Cycle

The clock input (OSC1/CLKIN pin) is internally divided

by four to generate four non-overlapping quadrature

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

program counter is incremented every Q1, and the

instruction is fetched from program memory and

latched into instruction register in Q4. It is decoded

and executed during the following Q1 through Q4. The

clocks and instruction execution ﬂow is shown in

Figure 3-2 and Example 3-1.

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

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

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.

1. MOVLW 03H

Fetch 1

Execute 1

2. MOVWF GPIO

Fetch 2

Execute 2

3. CALL

SUB_1

Fetch 3

Execute 3

4. BSF

GPIO, BIT1

Fetch 4

Flush

Fetch SUB_1 Execute SUB_1

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