DSPIC30F2023 Microchip Technology Inc., DSPIC30F2023 Datasheet - Page 45

• Manufacturer Part Number: DSPIC30F2023
• Description: 28/44-pin Dspic30f1010/202x Enhanced Flash Smps 16-bit Digital Signal Controller
• Manufacturer: Microchip Technology Inc.

4.2
Modulo addressing is a method of providing an auto-
mated means to support circular data buffers using
hardware. The objective is to remove the need for soft-
ware 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 pro-
gram space (since the data pointer mechanism is essen-
tially 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. How-
ever, 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 cer-
tain 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 buff-
ers 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 bound-
ary checks will be performed on both the lower and
upper address boundaries).
© 2006 Microchip Technology Inc.
Modulo Addressing
Preliminary
4.2.1
The modulo addressing scheme requires that a
starting and an end address be specified and loaded
into the 16-bit modulo buffer address registers:
XMODSRT, XMODEND, YMODSRT and YMODEND
(see Table 3-3).
The length of a circular buffer is not directly specified. It
is determined by the difference between the corre-
sponding start and end addresses. The maximum
possible length of the circular buffer is 32K words
(64 Kbytes).
4.2.2
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 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 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>.
Note:
dsPIC30F1010/202X
START AND END ADDRESS
Y-space modulo addressing EA calcula-
tions assume word sized data (LSb of
every EA is always clear).
W ADDRESS REGISTER
SELECTION
DS70178C-page 43