DSPIC33FJ64GP706 datasheet(65/322 Pages) MICROCHIP

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DS70286C-page 63

dsPIC33FJXXXGPX06/X08/X10

can operate on any W register pointer. However, it is not

advisable to use W14 or W15 for Modulo Addressing

since these two registers are used as the Stack Frame

Pointer and Stack Pointer, respectively.

In general, any particular circular buffer can only be

configured to operate in one direction as there are

certain restrictions on the buffer start address (for incre-

menting buffers), or end address (for decrementing

buffers), based upon the direction of the buffer.

The only exception to the usage restrictions is for

buffers which have a power-of-2 length. As these

buffers satisfy the start and end address criteria, they

may operate in a bidirectional mode (i.e., address

boundary checks will be performed on both the lower

and upper address boundaries).

4.4.1

START AND END ADDRESS

The Modulo Addressing scheme requires that a starting

and ending address be specified and loaded into the

16-bit Modulo Buffer Address registers: XMODSRT,

XMODEND,

YMODSRT

and

YMODEND

(see

Table 4-1).

The length of a circular buffer is not directly specified. It

is determined by the difference between the

corresponding start and end addresses. The maximum

possible length of the circular buffer is 32K words

(64 Kbytes).

4.4.2

W ADDRESS REGISTER

SELECTION

The Modulo and Bit-Reversed Addressing Control

as a W register field to specify the W Address registers.

The XWM and YWM fields select which registers will

operate with Modulo Addressing. If XWM = 15, X RAGU

and X WAGU Modulo Addressing is disabled. Similarly, if

YWM = 15, Y AGU Modulo Addressing is disabled.

The X Address Space Pointer W register (XWM), to

which Modulo Addressing is to be applied, is stored in

MODCON<3:0> (see Table 4-1). Modulo Addressing is

enabled for X data space when XWM is set to any value

other than ‘15’ and the XMODEN bit is set at

MODCON<15>.

The Y Address Space Pointer W register (YWM) to

which Modulo Addressing is to be applied is stored in

MODCON<7:4>. Modulo Addressing is enabled for Y

data space when YWM is set to any value other than

‘15’ and the YMODEN bit is set at MODCON<14>.

FIGURE 4-7:

MODULO ADDRESSING OPERATION EXAMPLE

Note:

space

Modulo

Addressing

calculations assume word sized data (LSb

of every EA is always clear).

0x1100

0x1163

Start Addr = 0x1100

End Addr = 0x1163

Length = 0x0032 words

Byte

Address

MOV

#0x1100, W0

MOV

W0, XMODSRT

;set modulo start address

MOV

#0x1163, W0

MOV

W0, MODEND

;set modulo end address

MOV

#0x8001, W0

MOV

W0, MODCON

;enable W1, X AGU for modulo

MOV

#0x0000, W0

;W0 holds buffer fill value

MOV

#0x1110, W1

;point W1 to buffer

AGAIN, #0x31

;fill the 50 buffer locations

MOV

W0, [W1++]

;fill the next location

AGAIN:

INC

W0, W0

;increment the fill value

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