PIC17C44 datasheet(48/241 Pages) MICROCHIP

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PIC17C4X

DS30412C-page 48

7.3

Table Reads

The table read allows the program memory to be read.

This allows constant data to be stored in the program

memory space, and retrieved into data memory when

needed. Example 7-2 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 + 1. The ﬁrst read loads the data into

the latch, and can be considered a dummy read

(unknown data loaded into 'f'). INDF0 should be con-

ﬁgured for either auto-increment or auto-decrement.

EXAMPLE 7-2:

TABLE READ

MOVLW

HIGH (TBL_ADDR) ; Load the Table

MOVWF

TBLPTRH

;

address

MOVLW

LOW (TBL_ADDR)

;

MOVWF

TBLPTRL

;

TABLRD

0,0,DUMMY

; Dummy read,

;

Updates TABLATCH

TLRD

1, INDF0

; Read HI byte

;

of TABLATCH

TABLRD

0,1,INDF0

; Read LO byte

;

of TABLATCH and

;

Update TABLATCH

FIGURE 7-7:

TABLRD TIMING

FIGURE 7-8:

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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