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dsPIC30F3010/3011

DS70141F-page 30

2010 Microchip Technology Inc.

FIGURE 3-4:

PROGRAM DATA TABLE ACCESS (MSB)

3.1.2

DATA ACCESS FROM PROGRAM

MEMORY USING PROGRAM SPACE

VISIBILITY

The upper 32 Kbytes of data space may optionally be

mapped into any 16K word program space page. This

provides transparent access of stored constant data

from X data space, without the need to use special

instructions (i.e., TBLRDL/H, TBLWTL/H instructions).

Program space access through the data space occurs

if the MSb of the data space, EA, is set and program

space visibility is enabled, by setting the PSV bit in the

Core Control register (CORCON). The functions of

CORCON

are

discussed

Section 2.4

“DSP

Engine”.

Data accesses to this area add an additional cycle to

the instruction being executed, since two program

memory fetches are required.

Note that the upper half of addressable data space is

always part of the X data space. Therefore, when a

DSP operation uses program space mapping to access

this memory region, Y data space should typically

contain state (variable) data for DSP operations,

whereas X data space should typically contain

coefficient (constant) data.

Although each data space address, 0x8000 and higher,

maps directly into a corresponding program memory

address, as shown in Figure 3-5, only the lower 16 bits

of the 24-bit program word are used to contain the data.

The upper 8 bits should be programmed to force an ille-

gal instruction to maintain machine robustness. Refer

to the “16-bit MCU and DSC Programmer’s Reference

Manual” (DS70157) for details on instruction encoding.

Note that by incrementing the PC by 2 for each

program memory word, the 15 LSbs of data space

addresses directly map to the 15 LSbs in the

corresponding

program

space

addresses.

The

remaining bits are provided by the Program Space

Visibility Page register, PSVPAG<7:0>, as shown in

Figure 3-5.

For instructions that use PSV which are executed

outside a REPEAT loop:

The following instructions will require one instruc-

tion cycle in addition to the specified execution

time:

- MAC class of instructions with data operand

prefetch

- MOV instructions

- MOV.D instructions

All other instructions will require two instruction

cycles in addition to the specified execution time

of the instruction.

For instructions that use PSV which are executed

inside a REPEAT loop:

The following instances will 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.

PC Address

0x000000

0x000002

0x000004

0x000006

00000000

Program Memory

‘Phantom’ Byte

(Read as ‘0’)

TBLRDH.W

TBLRDH.B (Wn<0> = 1)

TBLRDH.B (Wn<0> = 0)

Note:

PSV access is temporarily disabled during

table reads/writes.

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