PIC24F32KA302 datasheet(55/364 Pages) MICROCHIP

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DS39995D-page 55

PIC24FV32KA304 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 (in PIC24FV16KA3XX

devices) and a 32K word page (in PIC24FV32KA3XX

devices) 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 the 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 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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