Look-up table facilitates bit flipping

Brad Bierschenk, High End Systems, Austin, TX -- EDN, 11/23/2000

In certain instances in embedded software, it becomes necessary to flip the order of bits in a byte, so that B7:B0 becomes B0:B7. For example, this feature could be useful with a synchronous serial port that does not allow programmed selection of bit order (MSB first or LSB first) for its shift register. If a device to which the processor sends data expects one bit order but the serial port can provide only the other bit order, you must use a software method to translate the data. One solution to this problem is to provide a look-up table in ROM, in which the value of each byte in the table is offset into the table, but with bit order reversed. In other words, the first byte is offset 0 (00000000b), the second byte is offset 1 (10000000b), the third byte is offset 2 (01000000b), and so on.

The program merely needs to load the value to be translated into a register that can serve as an offset, index the look-up table, and load the corresponding value from the index+offset location. This design uses the Philips 80xC51 architecture as an example. Listing 1 shows a code segment for the µC. You can use the µC's 16-bit DPTR (data pointer) plus an 8-bit offset in its accumulator to load the accumulator with a byte value. This solution to bit flipping is dynamically more efficient than rotating a byte location through carry bits or other possible solutions. However, it's not the most statically efficient solution, because it requires 256 bytes of ROM for the look-up table.