Datasheet

PIC17C7XX
DS30289C-page 64  1998-2013 Microchip Technology Inc.
8.3 Table Reads
The table read allows the program memory to be read.
This allows constants to be stored in the program mem-
ory space and retrieved into data memory when
needed. Example 8-2 reads the 16-bit value at program
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 con-
sidered a dummy read (unknown data loaded into 'f').
INDF0 should be configured for either auto-increment
or auto-decrement.
EXAMPLE 8-2: TABLE READ
FIGURE 8-7: TABLRD TIMING
FIGURE 8-8: TABLRD TIMING (CONSECUTIVE TABLRD INSTRUCTIONS)
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
Q1 Q2
Q3
Q4
Q1 Q2
Q3
Q4
Q1 Q2
Q3
Q4
Q1 Q2
Q3
Q4
AD15:AD0
Instruction
Fetched
Instruction
Executed
ALE
OE
WR
TABLRD INST (PC+1)
INST (PC+2)
INST (PC-1)
TABLRD cycle1
TABLRD cycle2 INST (PC+1)
Data read cycle
PC 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
OE
WR
INST (PC-1) TABLRD1 cycle1 TABLRD1 cycle2 TABLRD2 cycle1
TABLRD2 cycle2
Data read cycle Data read cycle
'1'
PC
PC+1
PC+2
PC+3
TBL1
Data in 1
TBL2 Data in 2