PIC18F258 Datasheet, PDF (43/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

PIC18FXX8
4.5 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, 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
are shown in Figure 4-4.
FIGURE 4-4:
CLOCK/INSTRUCTION CYCLE
OSC1
Q1
Q2
Q3
Q4
PC
OSC2/CLKO
(RC Mode)
Q1 Q2 Q3 Q4
PC
Fetch INST (PC)
Execute INST (PC-2)
Q1 Q2 Q3 Q4
PC+2
Fetch INST (PC+2)
Execute INST (PC)
Q1 Q2 Q3 Q4
PC+4
Fetch INST (PC+4)
Execute INST (PC+2)
Internal
Phase
Clock
4.6 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 program counter to change (e.g., GOTO),
two cycles are required to complete the instruction
(Example 4-2).
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).
4.7 Instructions in Program Memory
The program memory is addressed in bytes. Instruc-
tions are stored as two bytes or four bytes in program
memory. The Least Significant Byte of an instruction
word is always stored in a program memory location
with an even address (LSB = ’0’). Figure 4-3 shows an
example of how instruction words are stored in the pro-
gram memory. To maintain alignment with instruction
boundaries, the PC increments in steps of 2 and the
LSB will always read ’0’ (see Section 4.4).
The CALL and GOTO instructions have an absolute pro-
gram memory address embedded into the instruction.
Since instructions are always stored on word bound-
aries, the data contained in the instruction is a word
address. The word address is written to PC<20:1>,
which accesses the desired byte address in program
memory. Instruction #2 in Example 4-3 shows how the
instruction “GOTO 000006h” is encoded in the program
memory. Program branch instructions that encode a
relative address offset operate in the same manner.
The offset value stored in a branch instruction repre-
sents the number of single word instructions by which
the PC will be offset. Section 25.0 provides further
details of the instruction set.
 2002 Microchip Technology Inc.
Preliminary
DS41159B-page 41