PIC24FJ16MC101_12 Datasheet, PDF (58/350 Pages) Microchip Technology – 16-bit Microcontrollers (up to 32 KB Flash and 2 KB SRAM)

PIC24FJ16MC101/102 AND PIC24FJ32MC101/102/104
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 to stored
constant data from the data space without the need to
use special instructions (such as TBLRDL and
TBLRDH).
Program space access through the data space occurs
if the MSb 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 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 to this area add a 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 4-9), 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, and are executed inside
a REPEAT loop, these instances 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 using PSV to access data, to execute in a
single cycle.
FIGURE 4-9:
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 _0x000000
0x010000
0x018000
Data Space
0x0000
Data EA<14:0>
0x8000
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.
DS39997C-page 58
Preliminary
© 2011-2012 Microchip Technology Inc.