D301 datasheet(55/416 Pages) MICROCHIP

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DS30010118D-page 55

PIC24FJ256GA705 FAMILY

4.2.5.1

Data Read from EDS

In order to read the data from the EDS space, first, an

Address Pointer is set up by loading the required EDS

page number into the DSRPAG register and assigning

the offset address to one of the W registers. Once the

above assignment is done, the EDS window is enabled

by setting bit 15 of the Working register which is

assigned with the offset address; then, the contents of

the pointed EDS location can be read.

Figure 4-4 illustrates how the EDS space address is

generated for read operations.

When the Most Significant bit (MSb) of EA is ‘1’ and

DSRPAG<9> = 0, the lower nine bits of DSRPAG are

concatenated to the lower 15 bits of EA to form a 24-bit

EDS space address for read operations.

Example 4-1 shows how to read a byte, word and

double word from EDS.

FIGURE 4-4:

EDS ADDRESS GENERATION FOR READ OPERATIONS

EXAMPLE 4-1:

EDS READ CODE IN ASSEMBLY

Note:

All read operations from EDS space have

an overhead of one instruction cycle.

Therefore, a minimum of two instruction

cycles are required to complete an EDS

read. EDS reads under the REPEAT

instruction; the first two accesses take

three

cycles

and

the

subsequent

accesses take one cycle.

DSRPAG Reg

Select

15 Bits

9 Bits

24-Bit EA

Wn<0> is Byte Select

0 = Extended SRAM and EPMP

; Set the EDS page from where the data to be read

mov

#0x0002, w0

mov

w0, DSRPAG

;page 2 is selected for read

mov

#0x0800, w1

;select the location (0x800) to be read

bset

w1, #15

;set the MSB of the base address, enable EDS mode

;Read a byte from the selected location

mov.b

[w1++], w2

;read Low byte

mov.b

[w1++], w3

;read High byte

;Read a word from the selected location

mov

[w1], w2

;

;Read Double - word from the selected location

mov.d

[w1], w2

;two word read, stored in w2 and w3

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