Microchip Technology dsPIC30F - Page 76

dsPIC30F Family Reference Manual
DS70050C-page 3-16 © 2004 Microchip Technology Inc.
3.4.3 Bit-Reverse Modifier Value
The value loaded into the XBREV register is a constant that indirectly defines the size of the
bit-reversed data buffer. The XB modifier values used with common bit-reversed buffers are
summarized in Table 3-2.
Table 3-2: Bit-Reversed Address Modifier Values
The bit-reverse hardware modifies the W register address by performing a ‘reverse-carry’
addition of the W contents and the XB modifier constant. A reverse-carry addition is performed
by adding the bits from left-to-right instead of right-to-left. If a carry-out occurs in a bit location,
the carry out bit is added to the next bit location to the right. Example 3-5 demonstrates the
reverse-carry addition and subsequent W register values using 0x0008 as the XB modifier
value. Note that the XB modifier is shifted one bit location to the left to generate word address
values.
Buffer Size (Words) XB Bit-Reversed Address Modifier Value
32768 0x4000
16384 0x2000
8192 0x1000
4096 0x0800
2048 0x0400
1024 0x0200
512 0x0100
256 0x0080
128 0x0040
64 0x0020
32 0x0010
16 0x0008
8 0x0004
4 0x0002
2 0x0001
Note: Only the the bit-reversed modifier values shown will produce valid bit-reversed
address sequences.