Datasheet
PIC12C67X
DS30561C-page 10 1997-2013 Microchip Technology Inc.
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
OSC2/CLKOUT
(EXTRC and
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
INTRC modes)
All instructions are single cycle, except for any program branches. These take two cycles since the fetched
instruction is “flushed” from the pipeline while the new instruction is being fetched and then executed.
TCY0TCY1TCY2TCY3TCY4TCY5
1. MOVLW 55h
Fetch 1 Execute 1
2. MOVWF GPIO
Fetch 2 Execute 2
3. CALL SUB_1
Fetch 3 Execute 3
4. BSF GPIO, BIT3 (Forced NOP)
Fetch 4 Flush
5. Instruction @ address SUB_1
Fetch SUB_1 Execute SUB_1
3.1 Clocking Scheme/Instruction Cycle
The clock input (from OSC1) is internally divided by
four to generate four non-overlapping quadrature
clocks, namely Q1, Q2, Q3 and Q4. Internally, the pro-
gram counter (PC) is incremented every Q1, and the
instruction is fetched from the program memory and
latched into the instruction register in Q4. The instruc-
tion is decoded and executed during the following Q1
through Q4. The clocks and instruction execution flow
is shown in Figure 3-2.
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
(i.e., 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).