PIC24H datasheet(53/290 Pages) MICROCHIP

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Preliminary

DS70175D-page 53

PIC24H

3.4.3

READING DATA FROM PROGRAM

MEMORY USING PROGRAM SPACE

VISIBILITY

The upper 32 Kbytes of data space may optionally be

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

This option provides transparent access of stored con-

stant 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 Most Significant bit of the data space EA is ‘1’ and

program space visibility is enabled by setting the PSV

bit in the Core Control register (CORCON<2>). The

location of the program memory space to be mapped

into the data space is determined by the Program

Space Visibility Page register (PSVPAG). This 8-bit

16K words in program space. In effect, PSVPAG func-

tions as the upper 8 bits of the program memory

address, with the 15 bits of the EA functioning as the

lower bits. Note that 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 to 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 3-8), 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

require one instruction cycle in addition to the specified

execution time. All other instructions 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 3-8:

PROGRAM SPACE VISIBILITY OPERATION

Note:

PSV access is temporarily disabled during

table reads/writes.

PSVPAG

Data Space

Program Space

0x0000

0x8000

0xFFFF

0x000000

0x800000

0x010000

0x018000

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

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.

PSV Area

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