DSPIC30F4012 datasheet(39/238 Pages) MICROCHIP

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DS70135G-page 39

dsPIC30F4011/4012

4.2.1

START AND END ADDRESS

The Modulo Addressing scheme requires that a start and

end address be specified and loaded into the 16-bit Mod-

ulo Buffer Address registers: XMODSRT, XMODEND,

YMODSRT and YMODEND (see Table 3-3).

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

determined

the

difference

between

the

corresponding start and end addresses. The maximum

possible length of the circular buffer is 32K words

(64 Kbytes).

4.2.2

W ADDRESS REGISTER

SELECTION

The Modulo and Bit-Reversed Addressing Control reg-

ister, MODCON<15:0>, contains enable flags as well

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

The XWM and YWM fields select which registers oper-

ate with Modulo Addressing. If XWM = 15, X RAGU and

X WAGU Modulo Addressing are 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 3-3). 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 4-1:

MODULO ADDRESSING OPERATION EXAMPLE

Note:

Y space Modulo Addressing EA calcula-

tions 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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