M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Basic Logic Elements
Logic Levels
Freescale Semiconductor, Inc.
This chapter begins with a close look at the requirements for logic-level
voltages. Transistors and interconnections for a typical CMOS
(complementary metal-oxide semiconductor) microcontroller are
discussed. A simple inverter, NAND gate, and NOR gate are explained.
Finally, a transmission gate, a three-state buffer, and a flip-flop circuit
are described. Virtually any part of a microcontroller can be explained in
terms of these few simple logic elements.
Earlier, in the discussion of what a microcontroller is, we said a level of
approximately 0 volts indicates a logic 0 and a voltage approximately
equal to the positive power source indicates a logic 1 signal. To be more
precise, there is a voltage level below which the microcontroller
manufacturer guarantees that a signal will be recognized as a valid
logic 0. Similarly, there is a voltage level above which the microcontroller
manufacturer guarantees that a signal will be recognized as a valid
logic 1. When designing a microcontroller system, be sure that all signals
conform to these specified limits, even under worst-case conditions.
Most modern microcontrollers use a technology called complementary
metal-oxide semiconductor (CMOS). This means the circuits include
both N-type and P-type transistors. Transistors will be explained in
greater detail later in this chapter.
In a typical CMOS circuit, a logic 0 input may be specified as 0.0 volts to
0.3 times VDD. If VDD is 5.0 volts, this translates to the range 0.0 to
1.5 volts. A logic 1 input may be specified as 0.7 times VDD to VDD. If
VDD is 5.0 volts, this translates to the range 3.5 to 5.0 volts.
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
38 Basic Logic Elements
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