PIC24FJ256GB108-I Datasheet, PDF (62/352 Pages) Microchip Technology – 64/80/100-Pin, 16-Bit Flash Microcontrollers with USB On-The-Go (OTG)

PIC24FJ256GB110 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 any 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 Most Significant bit of the data space EA is ‘1’, and
program space visibility is enabled by setting the PSV bit
in the CPU Control register (CORCON<2>). The loca-
tion of the program memory space to be mapped into the
data space is determined by the Program Space Visibil-
ity Page Address register (PSVPAG). 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. 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 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 locations 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.
Note: PSV access is temporarily disabled during
table reads/writes.
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
When CORCON<2> = 1 and EA<15> = 1:
PSVPAG
02
The data in the page
designated by
PSVPAG is mapped
into the upper half of
the data memory
space....
Program Space
23
15
0 _000000h
010000h
018000h
Data Space
0000h
8000h
Data EA<14:0>
800000h
PSV Area
FFFFh
...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.
DS39897C-page 62
 2009 Microchip Technology Inc.