Page 50 - Hacking Roomba

Chapter 2 — Interfacing Basics 31

Setting and Clearing Bits

Many parts of the ROI represent multiple binary values within a single byte. For command
data bytes, these bits are like mini-commands within the larger command, telling Roomba
to do one small thing or another. In the MOTORS command, for instance, you can turn on or
off any combination of the three cleaning motors.
Bytes are divided into 8 binary digits called bits. Each bit has a position in a byte
and value equal to 2 position . Bit positions are numbered from 0 to 7. The corresponding bit
values are 1,2,4,8,16,32,64,128, more commonly represented in hexadecimal as
0x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80. Sometimes a set of bits is called a bit field. A
byte is a bit field of 8 bits.

The act of turning on a bit (and thus turning on a motor in the case of the MOTORS com-
mand) is called setting a bit. Turning off a bit is called clearing a bit. Most of the time, you
can start with all bits cleared and then set the ones you want. In practice this means start-
ing with a value of zero and then joining together the bit values of the bits you want
to set.
For example, in the MOTORS command if you want to turn on only the main brush (bit 2, bit
value 0x04) and the side brush (bit 0, bit value 0x01), you can do this:
databyte = 0x04 | 0x01

The vertical bar (|) is the logical OR operator and works sort of like addition in this context.
You can also build up a byte incrementally using the “|=” operator:
databyte = 0
databyte |= 0x04
databate |= 0x01

The result is the same as before, but now you can add code to easily pick which statement
to run. Setting bits is similar but a little more complex, by using the logical NOT and logical
AND operators on a bit’s value. For example, to clear the side brush bit (and thus turn it
off), you would do this:
databyte &= ~0x01
This type of complex setting and clearing of bits isn’t used much in the ROI. Usually the
very first method of setting bits is used. For completeness, the general definitions in a C-like
language for setting and clearing a bit in a byte given the bit’s position are as follows:

#define setbit(byte,bit) (byte) |= (1 << (bit))
#define clrbit(byte,bit) (byte) &= ~(1 << (bit))