70592C Datasheet, PDF (62/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
4.4.3
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
register defines any one of 256 possible pages of
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, 0x8000 and higher,
maps directly into a corresponding program memory
address (see Figure 4-8), 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.
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
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 4-8:
PROGRAM SPACE VISIBILITY OPERATION
When CORCON<2> = 1 and EA<15> = 1:
PSVPAG
02
The data in the page
designated by PSV-
PAG is mapped into
the upper half of the
data memory
space...
Program Space
23
15
0 _0x000000
0x010000
0x018000
Data Space
0x0000
0x8000
Data EA<14:0>
0x800000
PSV Area
0xFFFF
...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.
DS70592C-page 62
© 2011 Microchip Technology Inc.