From the avr-libc user manual (get it here
11.6 What is all this _BV() stuff about?
When performing low-level output work, which is a very central point in microcontroller
programming, it is quite common that a particular bit needs to be set or cleared
in some IO register. While the device documentation provides mnemonic names for
the various bits in the IO registers, and the AVR device-specific IO definitions reflect
these names in definitions for numerical constants, a way is needed to convert a bit
number (usually within a byte register) into a byte value that can be assigned directly
to the register. However, sometimes the direct bit numbers are needed as well (e. g. in
an SBI() instruction), so the definitions cannot usefully be made as byte values in the
So in order to access a particular bit number as a byte value, use the _BV() macro.
Of course, the implementation of this macro is just the usual bit shift (which is done
by the compiler anyway, thus doesn’t impose any run-time penalty), so the following
_BV(3) => 1 << 3 => 0x08
However, using the macro often makes the program better readable.
"BV" stands for "bit value", in case someone might ask you. :-)
Example: clock timer 2 with full IO clock (CS2x = 0b001), toggle OC2 output on
compare match (COM2x = 0b01), and clear timer on compare match (CTC2 = 1). Make
OC2 (PD7) an output.
TCCR2 = _BV(COM20)|_BV(CTC2)|_BV(CS20);
DDRD = _BV(PD7);
So, the following statement clears the TX_BIT in the TX_PORT register. Refer to the datasheet to understand the register and bit mnemonics. Not sure which datasheet to refer you to, as TX_PORT and TX_BIT aren't in the few AVR datasheets that I'm familiar with.
TX_PORT &= ~_BV(TX_BIT)