PIC12F508/509/16F505
3.1
Clocking Scheme/Instruction
Cycle
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 take another instruction
cycle. However, due to the pipelining, each instruction
effectively executes in one cycle. If an instruction
causes the PC to change (e.g., GOTO), then two cycles
are required to complete the instruction (Example 3-1).
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 PC
is incremented every Q1 and the instruction is fetched
from program memory and latched into the 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 3-3 and Example 3-1.
A fetch cycle begins with the 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 3-3:
CLOCK/INSTRUCTION CYCLE
Q2
Q3
Q4
Q2
Q3
Q4
Q2
Q3
Q4
Q1
Q1
Q1
OSC1
Q1
Q2
Q3
Q4
PC
Internal
phase
clock
PC
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)
EXAMPLE 3-1:
INSTRUCTION PIPELINE FLOW
1. MOVLW 03H
Fetch 1
Execute 1
Fetch 2
2. MOVWF PORTB
3. CALL SUB_1
Execute 2
Fetch 3
Execute 3
Fetch 4
4. BSF
PORTB, BIT1
Flush
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.
DS41236C-page 14
Preliminary
© 2007 Microchip Technology Inc.
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