DSPIC33EP64GS708 datasheet(58/484 Pages) MICROCHIP

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DS70005258C-page 58

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4.7.3

MODULO ADDRESSING

APPLICABILITY

Modulo Addressing can be applied to the Effective

Address (EA) calculation associated with any W

equal to:

• The upper boundary addresses for incrementing

buffers

• The lower boundary addresses for decrementing

buffers

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.

4.8

Bit-Reversed Addressing

Bit-Reversed Addressing mode 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 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.

4.8.1

BIT-REVERSED ADDRESSING

IMPLEMENTATION

Bit-Reversed Addressing mode is enabled when all of

these situations are met:

• BWMx bits (W register selection) in the MODCON

cannot be accessed using Bit-Reversed

Addressing)

• The BREN bit is set in the XBREV register

• The addressing mode used is Register Indirect

with Pre-Increment or Post-Increment

If the length of a bit-reversed buffer is M = 2N bytes,

the last ‘N’ bits of the data buffer start address must

be zeros.

XB<14:0> is the Bit-Reversed Addressing 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

does 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

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:

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 Addressing

correction is performed, but the contents of

the register remain unchanged.

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.

Note:

Modulo Addressing and Bit-Reversed

Addressing can be enabled simultaneously

using the same W register, but Bit-

Reversed Addressing operation will always

take precedence for data writes when

enabled.

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