4.4.4.1 Bit-Reversed Addressing Implementation

Bit-Reversed Addressing mode is enabled in any of these situations:

• BWMx bits (W register selection) in the MODCON register are any value other than ‘1111’ (the stack 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 = 2^N bytes, the last ‘N’ bits of the data buffer start address must be zeros.

The XB[14:0] bits are 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 Register Indirect Addressing mode is ignored. In addition, as word-sized data are 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.