M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Basic Logic Elements
Freescale Semiconductor, Inc.
Transmission Gates, Buffers, and Flip Flops
Microcontrollers include more-complex types of logic gates and
functional elements than those shown in the previous section. In this
section, we explore some of these more-complex structures. The first
two structures — transmission gate and three-state buffer — introduce
the idea of logically controlled high-impedance signals. The third — half
flip flop — introduces a structure that can maintain a signal at its output
even after the input signal has changed. Flip flops are vital for a
microcontroller to perform counting and sequencing tasks.
Transmission Gate Figure 7 shows the logic symbol, a truth table for a CMOS transmission
gate, and a CMOS equivalent circuit. When control input [3] is a logic 1,
the transmission gate is said to be on and whatever logic level is present
on the input [1] is also seen at the output [2]. When the control input [3]
is a logic 0, the transmission gate is said to be off and the output node
[2] appears to be disconnected from everything (high impedance or
Hi-Z).
[1] [2]
Control
[3]
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
[3]
Figure 7. CMOS Transmission Gate
Refer to the CMOS equivalent circuit at the right of Figure 7 for the
following discussion: When control input [3] is logic 0, the gate of N
transistor [6] will be logic 0 and the gate of P transistor [5] will be logic 1
(VDD). There is no voltage between ground and VDD that would cause P
transistor [5] or N transistor [6] to turn on, so there is no conduction
between the input [1] and the output [2]. Since output node [2] is
effectively isolated from everything, it is said to be high impedance.
44 Basic Logic Elements
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MOTOROLA
Input
[1]
Output
[2]
0 0 Hi-Z
0 1 Hi-Z
1 0 0
1 1 1
[1]
[4]
[3]
[5]
[6]
[2]