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dsPIC33FJ32GP204 Datasheet

  • dsPIC33FJ32GP204

  • High-Performance,16-bit Digital Signal Controllers

  • 270頁

  • Microchip   Microchip

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dsPIC33FJ32GP202/204 and dsPIC33FJ16GP304
3.4
Modulo Addressing
3.4.1
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 3-1).
Note:
Y space Modulo Addressing EA calcula-
tions assume word sized data (LSB of
every EA is always clear).
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 Stack Frame
Pointer and Stack Pointer, 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 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 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).
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).
3.4.2
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 the registers that will
operate with Modulo Addressing:
鈥?If XWM =
15,
X RAGU and X WAGU Modulo
Addressing is disabled.
鈥?If YWM =
15,
Y AGU Modulo Addressing is dis-
abled.
FIGURE 3-5:
Byte
Address
MODULO ADDRESSING OPERATION EXAMPLE
MOV
MOV
MOV
MOV
MOV
MOV
MOV
MOV
#0x1100, W0
W0, XMODSRT
#0x1163, W0
W0, MODEND
#0x8001, W0
W0, MODCON
#0x0000, W0
#0x1110, W1
;set modulo start address
;set modulo end address
;enable W1, X AGU for modulo
;W0 holds buffer fill value
;point W1 to buffer
;fill the 50 buffer locations
;fill the next location
;increment the fill value
0x1100
DO
AGAIN, #0x31
MOV
W0, [W1++]
AGAIN: INC W0, W0
0x1163
Start Addr = 0x1100
End Addr = 0x1163
Length = 0x0032 words
DS70290A-page 42
Preliminary
漏
2007 Microchip Technology Inc.

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