Datasheet

 2009-2012 Microchip Technology Inc. DS70594D-page 65
dsPIC33FJXXXMCX06A/X08A/X10A
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 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
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 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 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
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
DO AGAIN, #0x31 ;fill the 50 buffer locations
MOV W0, [W1++] ;fill the next location
AGAIN: INC W0, W0 ;increment the fill value