DSPIC33FJ12GP201TESLASHML datasheet(43/242 Pages) MICROCHIP

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Preliminary

DS70264B-page 41

dsPIC33FJ12GP201/202

3.4.3

MODULO ADDRESSING

APPLICABILITY

Modulo Addressing can be applied to the EA

calculation associated with any W register.

Address boundaries check for addresses 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

also check for addresses less than or greater than

these addresses. Address changes can, therefore,

jump beyond boundaries and still be adjusted correctly.

3.5

Bit-Reversed Addressing

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

IMPLEMENTATION

Bit-Reversed Addressing mode is enabled in any of

these situations:

• BWM 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 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 Register 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:

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 cor-

rection 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

should

not

enabled

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.

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