M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
Basic Logic Elements
Freescale Semiconductor, Inc.
Three-State Buffer Figure 9 shows the logic symbol, a CMOS equivalent circuit, and a truth
table for a CMOS three-state buffer. When control input [3] is a logic 0,
the buffer is said to be off and output [2] is an isolated high impedance
node. When control input [3] is a logic 1, the buffer is said to be on and
whatever logic level is present on the input [1] is also seen at the output
[2].
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
Table 8. Data Multiplexer Operation
Select
Input Transmission Gate Output
[3]
[1] [2] [6] [7] [4]
0 0 0 Off On 0
0 0 1 Off On 1
0 1 0 Off On 0
0 1 1 Off On 1
1 0 0 On Off 0
1 0 1 On Off 0
1 1 0 On Off 1
1 1 1 On Off 1
Control
[3]
[1] [2]
Input
[1]
[3]
Output
[2]
0 0 Hi-Z
0 1 Hi-Z
1 0 0
1 1 1
Figure 9. Three-State Buffer
Refer to the CMOS equivalent circuit at the right of Figure 9 and to
Table 9 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]
through inverter [9], will be logic 1 (VDD), so both transistors [5] and [6]
46 Basic Logic Elements
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MOTOROLA
[1]
[4]
[3]
[9]
V DD
[7]
[5]
[6]
[8]
[2]