DSPIC30F datasheet(39/248 Pages) MICROCHIP

39 / 248 page

 2004 Microchip Technology Inc.

Preliminary

DS70083G-page 37

dsPIC30F

FIGURE 3-3:

PROGRAM DATA TABLE ACCESS (MS BYTE)

3.1.2

PROGRAM SPACE VISIBILITY

FROM DATA SPACE

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 MS bit 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 in Section 2.5, 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 con-

tain 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 (see Figure 3-4), 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 illegal instruction to maintain machine robust-

ness. Refer to the Programmer’s Reference Manual

(DS70030) for details on instruction encoding.

Note that by incrementing the PC by 2 for each

program memory word, the LS 15 bits of data space

addresses directly map to the LS 15 bits in the corre-

sponding program space addresses. The remaining

bits are provided by the Program Space Visibility Page

For instructions that use PSV which are executed

outside a REPEAT loop:

• The following instructions will require one

instruction cycle in addition to the specified

execution time:

- MAC class of instructions with data operand

pre-fetch

- 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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