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2011 Microchip Technology Inc.

DS31037B-page 46

PIC24F16KL402 FAMILY

4.3.3

READING DATA FROM PROGRAM

MEMORY USING PROGRAM SPACE

VISIBILITY

The upper 32 Kbytes of data space may optionally be

mapped into a 16K word page of the program space.

This provides transparent access of stored constant

data from the data space without the need to use

special instructions (i.e., TBLRDL/H).

Program space access through the data space occurs

if the MSb of the data space EA is ‘1’ and PSV is

enabled by setting the PSV bit in the CPU Control

(CORCON<2>) register. The location of the program

memory space to be mapped into the data space is

determined by the Program Space Visibility Page

Address (PSVPAG) register. This 8-bit register defines

any one of 256 possible pages of 16K words in

program space. In effect, PSVPAG functions as the

upper 8 bits of the program memory address, with

15 bits of the EA functioning as the lower bits.

By incrementing the PC by 2 for each program memory

word, the lower 15 bits of data space addresses directly

map to the lower 15 bits in the corresponding program

space addresses.

Data reads from this area add an additional cycle to the

instruction being executed, since two program memory

fetches are required.

Although each data space address, 8000h and higher,

maps directly into a corresponding program memory

address (see Figure 4-7), only the lower 16 bits of the

24-bit program word are used to contain the data. The

upper 8 bits of any program space location, used as

data, should be programmed with ‘1111 1111’ or ‘0000

0000

’ to force a NOP. This prevents possible issues

should the area of code ever be accidentally executed.

For operations that use PSV and are executed outside of

a REPEAT loop, the MOV and MOV.D instructions will

require one instruction cycle, in addition to the specified

execution time. All other instructions will require two

instruction cycles in addition to the specified execution

time.

For operations that use PSV, which are executed inside

a REPEAT loop, there will be some instances that

require two instruction cycles, in addition to the

specified execution time of the instruction:

Execution in the first iteration

Execution in the last iteration

Execution prior to exiting the loop due to an

interrupt

Execution upon re-entering the loop after an

interrupt is serviced

Any other iteration of the REPEAT loop will allow the

instruction accessing data, using PSV, to execute in a

single cycle.

FIGURE 4-7:

PROGRAM SPACE VISIBILITY OPERATION

Note:

PSV access is temporarily disabled during

table reads/writes.

PSVPAG

Data Space

Program Space

0000h

8000h

FFFFh

000000h

800000h

002BFEh

When CORCON<2> = 1 and EA<15> = 1:

PSV Area

The data in the page

designated by

PSVPAG is mapped

into the upper half of

the data memory

space....

Data EA<14:0>

...while the lower 15 bits

of the EA specify an exact

address within the PSV

area. This corresponds

exactly to the same lower

15 bits of the actual

program space address.

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