Datasheet

PIC12F519
DS41319B-page 12 © 2008 Microchip Technology Inc.
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 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 3-2 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 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).
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 3-2: CLOCK/INSTRUCTION CYCLE
EXAMPLE 3-1: INSTRUCTION PIPELINE FLOW
Q1
Q2 Q3 Q4
Q1
Q2 Q3 Q4
Q1
Q2 Q3 Q4
OSC1
Q1
Q2
Q3
Q4
PC
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)
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, 1
Fetch 4 Flush
Fetch SUB_1 Execute SUB_1