DSPIC33FJ06GS201TESLASHMM datasheet(75/346 Pages) MICROCHIP

75 / 346 page

Preliminary

DS70318D-page 73

dsPIC33FJ06GS101/X02 and dsPIC33FJ16GSX02/X04

4.4

Modulo Addressing

Modulo Addressing mode is a method used to provide

an automated means to support circular data buffers

using hardware. The objective is to remove the need

for software to perform data address boundary checks

when executing tightly looped code, as is typical in

many DSP algorithms.

Modulo Addressing can operate in either data or program

space (since the data pointer mechanism is essentially

the same for both). One circular buffer can be supported

in each of the X (which also provides the pointers into

program space) and Y data spaces. Modulo Addressing

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 be

configured to operate in only one direction as there are

certain restrictions on the buffer start address (for

incrementing

buffers),

end

address

(for

decrementing buffers), based upon the direction of the

buffer.

The only exception to the usage restrictions is for

buffers that have a power-of-two length. As these

buffers satisfy the start and end address criteria, they

can operate in a bidirectional mode (that is, address

boundary checks are 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

well as a W register field to specify the W Address

registers. The XWM and YWM fields select the

registers that will operate with Modulo Addressing:

• If XWM = 15, X RAGU and X WAGU Modulo

Addressing is disabled.

• 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

DSPIC33FJ06GS201TE/MM Datasheet(PDF) 75 Page - Microchip Technology

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