PIC18F2221 MICROCHIP [Microchip Technology], PIC18F2221 Datasheet - Page 59

• Manufacturer Part Number: PIC18F2221
• Description: 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology
• Manufacturer: MICROCHIP [Microchip Technology]

5.2
5.2.1
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; the instruction is 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 5-3.
FIGURE 5-3:
EXAMPLE 5-3:
© 2007 Microchip Technology Inc.
1. MOVLW 55h
2. MOVWF PORTB
3. BRA
4. BSF
5. Instruction @ address 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.
OSC2/CLKO
(RC mode)
PIC18 Instruction Cycle
SUB_1
CLOCKING SCHEME
PORTA, BIT3 (Forced NOP)
OSC1
PC
Q1
Q2
Q3
Q4
Q1
CLOCK/INSTRUCTION CYCLE
INSTRUCTION PIPELINE FLOW
Execute INST (PC – 2)
Fetch INST (PC)
Q2
Fetch 1
T
PC
CY
Q3
0
Q4
Execute 1
Fetch 2
T
CY
1
Q1
Preliminary
Fetch INST (PC + 2)
Execute INST (PC)
Q2
Execute 2
Fetch 3
PC + 2
T
CY
2
5.2.2
An “Instruction Cycle” consists of four Q cycles: Q1
through Q4. The instruction fetch and execute are
pipelined in such a manner that a fetch takes one
instruction cycle, while the decode and execute take
another instruction cycle. However, due to the pipe-
lining, 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 5-3).
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).
Q3
PIC18F4321 FAMILY
Q4
Execute 3
Fetch 4
T
CY
INSTRUCTION FLOW/PIPELINING
3
Q1
Execute INST (PC + 2)
Fetch INST (PC + 4)
Fetch SUB_1 Execute SUB_1
Flush (NOP)
Q2
PC + 4
T
CY
4
Q3
Q4
DS39689E-page 57
T
CY
Internal
Phase
Clock
5