DSPIC33EV64GM102-I/SO View Datasheet(PDF) - Microchip Technology
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DSPIC33EV64GM102-I/SO
Microchip Technology
DSPIC33EV64GM102-I/SO Datasheet PDF : 500 Pages
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4.5 Modulo Addressing
Modulo Addressing mode is a method of providing 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 SFP and SSP, respectively.
In general, any particular circular buffer can be config-
ured to operate in only one direction, as there are
certain restrictions on the buffer start address (for
incrementing buffers) or 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).
4.5.1 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).
Note:
Y Data Space Modulo Addressing EA
calculations assume word-sized data
(LSb of every EA is always clear).
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).
4.5.2
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 the registers that
operate with Modulo Addressing:
• If XWM = 1111, X RAGU and X WAGU Modulo
Addressing is disabled
• If YWM = 1111, 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 ‘1111’ and the XMODEN bit
(MODCON<15>) is set
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
‘1111’ and the YMODEN bit (MODCON<14>) is set.
Figure 4-15 shows an example of Modulo Addressing
operation.
FIGURE 4-15:
Byte
Address
0x1100
MODULO ADDRESSING OPERATION EXAMPLE
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
0x1163
MOV
#0x0000, W0 ;W0 holds buffer fill value
MOV
#0x1110, W1 ;point W1 to buffer
Start Addr = 0x1100
End Addr = 0x1163
Length = 32 Words
DO
AGAIN, #0x31
MOV
W0, [W1++]
AGAIN: INC W0, W0
;fill the 50 buffer locations
;fill the next location
;increment the fill value
DS70005144E-page 76
2013-2016 Microchip Technology Inc.
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