Datasheet

dsPIC33FJXXXGPX06A/X08A/X10A
DS70593D-page 70  2009-2012 Microchip Technology Inc.
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
register, 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 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 Ta bl e 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
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
DO AGAIN, #0x31 ;fill the 50 buffer locations
MOV W0, [W1++] ;fill the next location
AGAIN: INC W0, W0 ;increment the fill value