dsPIC33FJ32MC204 Microchip Technology, dsPIC33FJ32MC204 Datasheet - Page 47

• Manufacturer Part Number: dsPIC33FJ32MC204
• Description: (dsPIC33FJ16MC304 / dsPIC33FJ32MC20x) 16-bit Microcontrollers
• Manufacturer: Microchip Technology

3.4.3
Modulo Addressing can be applied to the Effective
Address (EA) calculation associated with any W
register. Address boundaries check for addresses
equal to:
• The upper boundary addresses for incrementing
• The lower boundary addresses for decrementing
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 can, therefore, jump beyond
boundaries and still be adjusted correctly.
3.5
Bit-Reversed Addressing mode is intended to simplify
data re-ordering for radix-2 FFT algorithms. It is
supported by the X AGU for data writes only.
The modifier, which can 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.
3.5.1
Bit-Reversed Addressing mode is enabled in any of
these situations:
• BWM bits (W register selection) in the MODCON
• The BREN bit is set in the XBREV register
• The addressing mode used is Register Indirect
© 2007 Microchip Technology Inc.
buffers
buffers
Note:
register are any value other than ‘15’ (the stack
cannot be accessed using Bit-Reversed
Addressing)
with Pre-Increment or Post-Increment
Bit-Reversed Addressing
MODULO ADDRESSING
APPLICABILITY
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 (such as [W7 +
W2]) is used, Modulo Address correction
is performed but the contents of the
register remain unchanged.
BIT-REVERSED ADDRESSING
IMPLEMENTATION
Preliminary
If the length of a bit-reversed buffer is M = 2
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.
When enabled, Bit-Reversed Addressing is executed
only 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 are generated
instead. When Bit-Reversed Addressing is active, the
W Address Pointer is always added to the address
modifier (XB), and the offset associated with the Regis-
ter Indirect Addressing mode is ignored. In addition, as
word-sized data is a requirement, the LSb of the EA is
ignored (and always clear).
If Bit-Reversed Addressing has already been enabled
by setting the BREN (XBREV<15>) bit, 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.
Note:
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.
Modulo Addressing and Bit-Reversed
Addressing
together. If an application attempts to do so,
Bit-Reversed Addressing will assume prior-
ity when active for the X WAGU and X
WAGU, Modulo Addressing will be dis-
abled. However, Modulo Addressing will
continue to function in the X RAGU.
should
not
DS70283B-page 45
be
enabled
N
bytes,