PIC17C7XX_13 Datasheet, PDF (64/306 Pages) Microchip Technology – High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D

PIC17C7XX
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
MOVLW
MOVWF
MOVLW
MOVWF
TABLRD
TLRD
TABLRD
HIGH (TBL_ADDR) ; Load the Table
TBLPTRH
; address
LOW (TBL_ADDR) ;
TBLPTRL
;
0, 1, DUMMY ; Dummy read,
; Updates TABLATH
; Increments TBLPTR
1, INDF0
; Read HI byte
; of TABLATH
0, 1, INDF0 ; Read LO byte
; of TABLATL and
; Update TABLATH
; Increment TBLPTR
FIGURE 8-7:
TABLRD TIMING
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
AD15:AD0
PC
PC+1
TBL Data in
PC+2
Instruction
Fetched
Instruction
Executed
TABLRD
INST (PC+1)
INST (PC-1)
TABLRD cycle1
TABLRD cycle2
Data read cycle
ALE
OE
'1'
WR
INST (PC+2)
INST (PC+1)
FIGURE 8-8:
TABLRD TIMING (CONSECUTIVE TABLRD INSTRUCTIONS)
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
PC
PC+1
TBL1 Data in 1
PC+2
TBL2 Data in 2
PC+3
Instruction
Fetched
Instruction
Executed
TABLRD1
TABLRD2
INST (PC+2)
INST (PC+3)
INST (PC-1) TABLRD1 cycle1 TABLRD1 cycle2 TABLRD2 cycle1 TABLRD2 cycle2 INST (PC+2)
Data read cycle
Data read cycle
ALE
OE
'1'
WR
DS30289C-page 64
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