PIC18F2455-I/SP Microchip Technology Inc., PIC18F2455-I/SP Datasheet - Page 59

• Manufacturer Part Number: PIC18F2455-I/SP
• Description: Microcontroller; 24 KB Flash; 2048 RAM; 256 EEPROM; 24 I/O; 28-Pin-SPDIP
• Manufacturer: Microchip Technology Inc.

Specifications of PIC18F2455-I/SP

• A/d Inputs: 10-Channel, 10-Bit
• Comparators: 2
• Cpu Speed: 12 MIPS
• Eeprom Memory: 256 Bytes
• Input Output: 23
• Interface: I2C/SPI/USART/USB
• Memory Type: Flash
• Number Of Bits: 8
• Package Type: 28-pin SPDIP
• Programmable Memory: 24K Bytes
• Ram Size: 2K Bytes
• Speed: 48 MHz
• Timers: 1-8-bit, 3-16-bit
• Voltage, Range: 2-5.5 V
• Lead Free Status / Rohs Status: RoHS Compliant part Electrostatic Device

5.2
5.2.1
The microcontroller clock input, whether from an
internal or external source, is internally divided by four
to generate four non-overlapping quadrature clocks
(Q1, Q2, Q3 and Q4). Internally, the program counter is
incremented on every Q1; the instruction is fetched
from the program memory and latched into the Instruc-
tion Register (IR) during Q4. The instruction is decoded
and executed during the following Q1 through Q4. The
clocks and instruction execution flow are shown in
Figure 5-3.
FIGURE 5-3:
EXAMPLE 5-3:
© 2006 Microchip Technology Inc.
Note:
1. MOVLW 55h
2. MOVWF PORTB
3. BRA
4. BSF
5. Instruction @ address SUB_1
OSC2/CLKO
(RC mode)
PIC18 Instruction Cycle
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.
CLOCKING SCHEME
SUB_1
OSC1
PORTA, BIT3 (Forced NOP)
PC
Q1
Q2
Q3
Q4
Q1
CLOCK/INSTRUCTION CYCLE
INSTRUCTION PIPELINE FLOW
Execute INST (PC – 2)
Fetch INST (PC)
Q2
Fetch 1
PC
T
CY
Q3
0
Q4
Execute 1
Advance Information
Fetch 2
T
CY
Q1
1
Fetch INST (PC + 2)
Execute INST (PC)
Q2
Execute 2
Fetch 3
PC + 2
T
CY
Q3
5.2.2
An “Instruction Cycle” consists of four Q cycles: Q1
through Q4. The instruction fetch and execute are
pipelined in such a manner that a fetch takes one
instruction cycle, while the 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 5-3).
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).
2
Q4
Execute 3
Fetch 4
PIC18F2450/4450
T
CY
INSTRUCTION FLOW/PIPELINING
3
Q1
Execute INST (PC + 2)
Fetch INST (PC + 4)
Fetch SUB_1 Execute SUB_1
Flush (NOP)
Q2
PC + 4
T
CY
Q3
4
Q4
DS39760A-page 57
T
CY
Internal
Phase
Clock
5