PIC18F2510-ESLASHML datasheet(56/376 Pages) MICROCHIP

56 / 376 page

PIC18F2X1X/4X1X

DS39636D-page 58

5.1.2.4

Stack Full and Underflow Resets

Device Resets on stack overflow and stack underflow

conditions are enabled by setting the STVREN bit in

Configuration Register 4L. When STVREN is set, a full

or underflow will set the appropriate STKFUL or

STKUNF bit and then cause a device Reset. When

STVREN is cleared, a full or underflow condition will set

the appropriate STKFUL or STKUNF bit but not cause

a device Reset. The STKFUL or STKUNF bits are

cleared by the user software or a Power-on Reset.

5.1.3

FAST REGISTER STACK

A fast register stack is provided for the STATUS,

WREG and BSR registers, to provide a “fast return”

option for interrupts. The stack for each register is only

one level deep and is neither readable nor writable. It is

loaded with the current value of the corresponding reg-

ister when the processor vectors for an interrupt. All

interrupt sources will push values into the stack regis-

ters. The values in the registers are then loaded back

into their associated registers if the RETFIE, FAST

instruction is used to return from the interrupt.

If both low and high priority interrupts are enabled, the

stack registers cannot be used reliably to return from

low priority interrupts. If a high priority interrupt occurs

while servicing a low priority interrupt, the stack register

values stored by the low priority interrupt will be

overwritten. In these cases, users must save the key

registers in software during a low priority interrupt.

If interrupt priority is not used, all interrupts may use the

fast register stack for returns from interrupt. If no inter-

rupts are used, the fast register stack can be used to

restore the STATUS, WREG and BSR registers at the

end of a subroutine call. To use the fast register stack

for a subroutine call, a CALL label, FAST instruction

must be executed to save the STATUS, WREG and

BSR registers to the fast register stack. A

RETURN, FAST instruction is then executed to restore

these registers from the fast register stack.

Example 5-1 shows a source code example that uses

the fast register stack during a subroutine call and

return.

EXAMPLE 5-1:

FAST REGISTER STACK

CODE EXAMPLE

5.1.4

LOOK-UP TABLES IN PROGRAM

MEMORY

There may be programming situations that require the

creation of data structures, or look-up tables, in

program memory. For PIC18 devices, look-up tables

can be implemented in two ways:

• Computed GOTO

• Table Reads

5.1.4.1

Computed GOTO

A computed GOTO is accomplished by adding an offset

to the program counter. An example is shown in

Example 5-2.

A look-up table can be formed with an ADDWF PCL

instruction and a group of RETLW nn instructions. The

W register is loaded with an offset into the table before

executing a call to that table. The first instruction of the

called routine is the ADDWF PCL instruction. The next

instruction executed will be one of the RETLW nn

instructions that returns the value ‘nn’ to the calling

function.

The offset value (in WREG) specifies the number of

bytes that the program counter should advance and

should be multiples of 2 (LSb = 0).

In this method, only one data byte may be stored in

each instruction location and room on the return

address stack is required.

EXAMPLE 5-2:

COMPUTED GOTO USING

AN OFFSET VALUE

5.1.4.2

Table Reads and Table Writes

A better method of storing data in program memory

allows two bytes of data to be stored in each instruction

location.

Look-up table data may be stored two bytes per

program word by using table reads and writes. The

Table Pointer (TBLPTR) register specifies the byte

address and the Table Latch (TABLAT) register

contains the data that is read from or written to program

memory. Data is transferred to or from program

memory one byte at a time.

Table read and table write operations are discussed

further in Section 6.1 “Table Reads”.

CALL SUB1, FAST

;STATUS, WREG, BSR

;SAVED IN FAST REGISTER

;STACK

•

SUB1

•

RETURN, FAST

;RESTORE VALUES SAVED

;IN FAST REGISTER STACK

MOVF

OFFSET, W

CALL

TABLE

ORG

nn00h

TABLE

ADDWF

PCL

RETLW

nnh

RETLW

nnh

RETLW

nnh

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