DSPIC30F5015_07 Datasheet, PDF (40/230 Pages) Microchip Technology – High-Performance, 16-Bit Digital Signal Controllers

dsPIC30F5015/5016
4.2.3
MODULO ADDRESSING
APPLICABILITY
Modulo Addressing can be applied to the Effective
Address (EA) calculation associated with any W register.
It is important to realize that the address boundaries
check for addresses less than or greater than the upper
(for incrementing buffers) and lower (for decrementing
buffers) boundary addresses (not just equal to). Address
changes may, therefore, jump beyond boundaries and
still be adjusted correctly.
Note:
The modulo corrected Effective Address is
written back to the register only when Pre-
Modify or Post-Modify Addressing mode is
used to compute the Effective Address.
When an address offset (e.g., [W7 + W2])
is used, modulo address correction is per-
formed, but the contents of the register
remains unchanged.
4.3 Bit-Reversed Addressing
Bit-Reversed Addressing is intended to simplify data
reordering for radix-2 FFT algorithms. It is supported by
the X AGU for data writes only.
The modifier, which may be a constant value or register
contents, is regarded as having its bit order reversed.
The address source and destination are kept in normal
order. Thus, the only operand requiring reversal is the
modifier.
4.3.1
BIT-REVERSED ADDRESSING
IMPLEMENTATION
Bit-Reversed Addressing is enabled when:
1. BWM (W register selection) in the MODCON
register is any value other than 15 (the stack can
not be accessed using Bit-Reversed Addressing)
and
2. the BREN bit is set in the XBREV register and
3. the addressing mode used is Register Indirect
with Pre-Increment or Post-Increment.
If the length of a Bit-Reversed buffer is M = 2^N bytes,
then the last ‘N’ bits of the data buffer start address
must be zeros.
XB<14:0> is the Bit-Reversed Address modifier or
‘pivot point’ which is typically a constant. In the case of
an FFT computation, its value is equal to half of the FFT
data buffer size.
Note:
All bit-reversed EA calculations assume
word-sized data (LSb of every EA is
always clear). The XB value is scaled
accordingly to generate compatible (byte)
addresses.
When enabled, Bit-Reversed Addressing will only be
executed for Register Indirect with Pre-Increment or
Post-Increment Addressing and word-sized data writes.
It will not function for any other addressing mode or for
byte-sized data, and normal addresses will be generated
instead. When Bit-Reversed Addressing is active, the W
address pointer will always be added to the address
modifier (XB) and the offset associated with the Register
Indirect Addressing mode will be ignored. In addition, as
word-sized data is a requirement, the LSb of the EA is
ignored (and always clear).
Note:
Modulo Addressing and Bit-Reversed
Addressing should not be enabled
together. In the event that the user
attempts to do this, Bit-Reversed Address-
ing will assume priority when active for the
X WAGU, and X WAGU Modulo Address-
ing will be disabled. However, Modulo
Addressing will continue to function in the
X RAGU.
If Bit-Reversed Addressing has already been enabled
by setting the BREN (XBREV<15>) bit, then a write to
the XBREV register should not be immediately followed
by an indirect read operation using the W register that
has been designated as the Bit-Reversed Pointer.
FIGURE 4-2:
BIT-REVERSED ADDRESS EXAMPLE
Sequential Address
b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b4 b3 b2 b1 0
Bit Locations Swapped Left-to-Right
Around Center of Binary Value
b15 b14 b13 b12 b11 b10 b9 b8 b7 b6 b5 b1 b2 b3 b4 0
Bit-Reversed Address
Pivot Point
XB = 0x0008 for a 16-word Bit-Reversed Buffer
DS70149C-page 38
© 2007 Microchip Technology Inc.