MC9S12UF32 Card Reader Reference Design User Manual, Rev. 0.1
Freescale Semiconductor
39
CPU
— See “central processor unit (CPU).”
CPU12
— The CPU of the MC68HC12 Family.
CPU clock
— Bus clock select bits BCSP and BCSS in the clock select register (CLKSEL) determine
which clock drives SYSCLK for the main system, including the CPU and buses. When EXTALi
drives the SYSCLK, the CPU or bus clock frequency (f
o
) is equal to the EXTALi frequency divided
by 2.
CPU cycles
— A CPU cycle is one period of the internal bus clock, normally derived by dividing a crystal
oscillator source by two or more so the high and low times will be equal. The length of time
required to execute an instruction is measured in CPU clock cycles.
CPU registers
— Memory locations that are wired directly into the CPU logic instead of being part of the
addressable memory map. The CPU always has direct access to the information in these
registers. The CPU registers in an M68HC12 are:
•
A (8-bit accumulator)
•
B (8-bit accumulator)
–
D (16-bit accumulator formed by concatenation of accumulators A and B)
•
IX (16-bit index register)
•
IY (16-bit index register)
•
SP (16-bit stack pointer)
•
PC (16-bit program counter)
•
CCR (8-bit condition code register)
cycle time
— The period of the operating frequency: t
CYC
= 1/f
OP
.
D
— See “accumulators (A and B or D).”
decimal number system
— Base 10 numbering system that uses the digits zero through nine.
duty cycle
— A ratio of the amount of time the signal is on versus the time it is off. Duty cycle is usually
represented by a percentage.
ECT
— See “enhanced capture timer.”
EEPROM
— Electrically erasable, programmable, read-only memory. A nonvolatile type of memory that
can be electrically erased and reprogrammed.
EPROM
— Erasable, programmable, read-only memory. A nonvolatile type of memory that can be erased
by exposure to an ultraviolet light source and then reprogrammed.
enhanced capture timer (ECT)
— The HC12 Enhanced Capture Timer module has the features of the
HC12 Standard Timer module enhanced by additional features in order to enlarge the field of
applications.
