M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Basic Logic Elements
Simple Gates
Freescale Semiconductor, Inc.
The three most basic types of logic gates found in a microcontroller are
the inverter, the NAND gate, and the NOR gate. A logic designer uses
various combinations of these basic gates to form more-complex logic
circuits, such as those that add two binary numbers together. While this
textbook is not intended to teach logic design techniques, these circuits
are discussed to give you a better understanding of how a
microcontroller operates on digital information.
Inverter Figure 4 shows the inverter logic symbol, a truth table for an inverter,
and a CMOS equivalent circuit. When a logic-level signal (0 or 1) is
presented to the input [1] of an inverter, the opposite logic level appears
at its output [2].
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
Figure 4. CMOS Inverter
Refer to the CMOS equivalent circuit at the right of Figure 4 and to
Table 5 for the following discussion: When input [1] is a logic 0, N
transistor [4] is off and P transistor [3] is on, connecting output [2] to VDD
(logic 1). When input [1] is a logic 1, P transistor [3] is off and N transistor
[4] is on, connecting output [2] to ground (logic 0).
Input
[1]
[1] [2]
Table 5. Inverter Gate Operation
Transistor Output
[2]
[3] [4]
0 On Off Connected to V DD (1)
1 Off On Connected to ground (0)
40 Basic Logic Elements
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MOTOROLA
Input
[1]
Output
[2]
0 1
1 0
[1]
V DD
[3]
[4]
[2]