PIC17C752-08SLASHCL datasheet(62/328 Pages) MICROCHIP

62 / 328 page

PIC17C7XX

DS30289A-page 62

8.3

Table Reads

The table read allows the program memory to be read.

This allows constants to be stored in the program

memory space, and retrieved into data memory when

needed. Example 8-2 reads the 16-bit value at pro-

gram memory address TBLPTR.

After the dummy

byte has been read from the TABLATH, the TABLATH

is loaded with the 16-bit data from program memory

address TBLPTR, and then increments the TBLPTR

value. The ﬁrst read loads the data into the latch, and

can be considered a dummy read (unknown data

loaded into 'f'). INDF0 should be conﬁgured for either

auto-increment or auto-decrement.

EXAMPLE 8-2:

TABLE READ

MOVLW

HIGH (TBL_ADDR) ; Load the Table

MOVWF

TBLPTRH

;

address

MOVLW

LOW (TBL_ADDR)

;

MOVWF

TBLPTRL

;

TABLRD

0, 1, DUMMY

; Dummy read,

;

Updates TABLATH

;

Increments TBLPTR

TLRD

1, INDF0

; Read HI byte

;

of TABLATH

TABLRD

0, 1, INDF0

; Read LO byte

;

of TABLATL and

;

Update TABLATH

;

Increment TBLPTR

FIGURE 8-7:

TABLRD

TIMING

FIGURE 8-8:

TABLRD

TIMING (CONSECUTIVE TABLRD INSTRUCTIONS)

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

AD15:AD0

Instruction

fetched

Instruction

executed

ALE

TABLRD

INST (PC+1)

INST (PC+2)

INST (PC-1)

TABLRD cycle1

TABLRD cycle2

INST (PC+1)

Data read cycle

PC+1

TBL

Data in

PC+2

'1'

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

AD15:AD0

Instruction

fetched

Instruction

executed

TABLRD1

TABLRD2

INST (PC+2)

INST (PC+3)

INST (PC+2)

ALE

INST (PC-1)

TABLRD1 cycle1 TABLRD1 cycle2 TABLRD2 cycle1 TABLRD2 cycle2

Data read cycle

'1'

PC+1

PC+2

PC+3

TBL1 Data in 1

TBL2

Data in 2

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