PIC17C44 datasheet(41/241 Pages) MICROCHIP

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DS30412C-page 41

PIC17C4X

6.7

Program Counter Module

The Program Counter (PC) is a 16-bit register. PCL, the

low byte of the PC, is mapped in the data memory. PCL

is readable and writable just as is any other register.

PCH is the high byte of the PC and is not directly

addressable. Since PCH is not mapped in data or pro-

gram memory, an 8-bit register PCLATH (PC high latch)

is used as a holding latch for the high byte of the PC.

PCLATH is mapped into data memory. The user can

read or write PCH through PCLATH.

The 16-bit wide PC is incremented after each instruc-

tion fetch during Q1 unless:

• Modiﬁed by GOTO, CALL, LCALL, RETURN, RETLW,

or RETFIE instruction

• Modiﬁed by an interrupt response

• Due to destination write to PCL by an instruction

“Skips” are equivalent to a forced NOP cycle at the

skipped address.

Figure 6-11 and Figure 6-12 show the operation of the

program counter for various situations.

FIGURE 6-11: PROGRAM COUNTER

OPERATION

FIGURE 6-12: PROGRAM COUNTER USING

THE CALL AND GOTO

INSTRUCTIONS

Internal data bus <8>

PCLATH

PCH

PCL

5 4

8 7

Last write

to PCLATH

PCLATH

Opcode

PCH

PCL

Using Figure 6-11, the operations of the PC and

PCLATH for different instructions are as follows:

LCALL

instructions:

An 8-bit destination address is provided in the

instruction (opcode). PCLATH is unchanged.

PCLATH

→ PCH

Opcode<7:0>

→ PCL

Read instructions on PCL:

Any instruction that reads PCL.

PCL

→ data bus → ALU or destination

PCH

→ PCLATH

Write instructions on PCL:

Any instruction that writes to PCL.

8-bit data

→ data bus → PCL

PCLATH

→ PCH

Read-Modify-Write instructions on PCL:

Any instruction that does a read-write-modify

operation on PCL, such as ADDWF PCL.

Read:

PCL

→ data bus → ALU

Write:

8-bit result

→ data bus → PCL

PCLATH

→ PCH

RETURN

instruction:

PCH

→ PCLATH

Stack<MRU>

→ PC<15:0>

Using Figure 6-12, the operation of the PC and

PCLATH for GOTO and CALL instructions is a follows:

CALL

, GOTO instructions:

A 13-bit destination address is provided in the

instruction (opcode).

Opcode<12:0>

→ PC <12:0>

PC<15:13>

→ PCLATH<7:5>

Opcode<12:8>

→ PCLATH <4:0>

The read-modify-write only affects the PCL with the

result. PCH is loaded with the value in the PCLATH.

For example, ADDWF PCL will result in a jump within the

current page. If PC = 03F0h, WREG = 30h and

PCLATH = 03h before instruction, PC = 0320h after the

instruction. To accomplish a true 16-bit computed jump,

the user needs to compute the 16-bit destination

address, write the high byte to PCLATH and then write

the low value to PCL.

The following PC related operations do not change

PCLATH:

LCALL

, RETLW, and RETFIE instructions.

Interrupt vector is forced onto the PC.

Read-modify-write instructions on PCL (e.g.BSF

PCL

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