PIC16C745/765
3.1
Clocking Scheme/Instruction Cycle
3.2
Instruction Flow/Pipelining
The clock input feeds an on-chip PLL. The clock output
from the PLL (FINT) is internally divided by four to gen-
erate four non-overlapping quadrature clocks namely,
Q1, Q2, Q3 and Q4. Internally, the program counter
(PC) is incremented every Q1, the instruction is fetched
from the program memory and latched into the instruc-
tion register in Q4. The instruction is decoded and exe-
cuted during the following Q1 through Q4. The clocks
and instruction execution flow is shown in Figure 3-2.
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 (e.g., 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).
FIGURE 3-2: CLOCK/INSTRUCTION CYCLE
Q2
Q3
Q4
Q2
Q3
Q4
Q2
Q3
Q4
Q1
Q1
Q1
FINT
Q1
Internal
phase
clock
Q2
Q3
Q4
PC
PC
PC+1
PC+2
OSC2/CLKOUT
(EC mode)
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
TCY0
TCY1
TCY2
TCY3
TCY4
TCY5
1. MOVLW 55h
Fetch 1
Execute 1
Fetch 2
2. MOVWF PORTB
3. CALL SUB_1
Execute 2
Fetch 3
Execute 3
Fetch 4
4. BSF
PORTA, BIT3 (Forced NOP)
Flush
5. Instruction @ address SUB_1
Fetch SUB_1 Execute SUB_1
Note: 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.
1999 Microchip Technology Inc.
Advanced Information
DS41124A-page 13
MICROCHIP [ MICROCHIP ]
