PIC18F45K80-I Datasheet, PDF (106/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN and nanoWatt XLP Technology

PIC18F66K80 FAMILY
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:
OSC1
Q1
Q2
Q3
Q4
PC
OSC2/CLKO
(RC mode)
CLOCK/INSTRUCTION CYCLE
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC
Execute INST (PC – 2)
Fetch INST (PC)
PC + 2
Execute INST (PC)
Fetch INST (PC + 2)
Q1 Q2 Q3 Q4
PC + 4
Execute INST (PC + 2)
Fetch INST (PC + 4)
Internal
Phase
Clock
EXAMPLE 6-3: INSTRUCTION PIPELINE FLOW
1. MOVLW 55h
2. MOVWF PORTB
TCY0
Fetch 1
TCY1
Execute 1
Fetch 2
3. BRA SUB_1
4. BSF PORTA, BIT3 (Forced NOP)
5. Instruction @ address SUB_1
TCY2
Execute 2
Fetch 3
TCY3
TCY4
TCY5
Execute 3
Fetch 4
Flush (NOP)
Fetch SUB_1 Execute SUB_1
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.
DS39977F-page 106
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