Glossary
68HC(9)12DG128 Rev 1.0
418
Glossary
MOTOROLA
pointer
— Pointer register. An index register is sometimes called a pointer register because its
contents are used in the calculation of the address of an operand, and therefore points to
the operand.
polarity
— The two opposite logic levels, logic 1 and logic 0, which correspond to two different
voltage levels, V
DD
and V
SS
.
polling
— Periodically reading a status bit to monitor the condition of a peripheral device.
port
— A set of wires for communicating with off-chip devices.
prescaler
— A circuit that generates an output signal related to the input signal by a fractional
scale factor such as 1/2, 1/8, 1/10 etc.
program
— A set of computer instructions that cause a computer to perform a desired operation
or operations.
program counter (PC)
— A 16-bit register in the CPU. The PC register holds the address of the
next instruction or operand that the CPU will use.
pull
— An instruction that copies into the accumulator the contents of a stack RAM location. The
stack RAM address is in the stack pointer.
pullup
— A transistor in the output of a logic gate that connects the output to the logic 1 voltage
of the power supply.
pulse-width
— The amount of time a signal is on as opposed to being in its off state.
pulse-width modulation (PWM)
— Controlled variation (modulation) of the pulse width of a
signal with a constant frequency.
push
— An instruction that copies the contents of the accumulator to the stack RAM. The stack
RAM address is in the stack pointer.
PWM period
— The time required for one complete cycle of a PWM waveform.
RAM
— Random access memory. All RAM locations can be read or written by the CPU. The
contents of a RAM memory location remain valid until the CPU writes a different value or
until power is turned off.
RC circuit
— A circuit consisting of capacitors and resistors having a defined time constant.
read
— To copy the contents of a memory location to the accumulator.
register
— A circuit that stores a group of bits.