ECE 171: Introduction to Digital Circuits

Fall 1999

Rev: 10.15.99

Lecture Notes 5

Last Time

• UNIX Primer
• Device Labeling
• Reading Data Sheets
• Noise Margins
• Fanout

This Time

• Data Sheets
• Noise Margins
• Fanout
• Propagation Delays
• Tristate Outputs
• Buffers

Connecting Devices of Different Types

Noise Margins

• The upper noise margin, NM_U, is given by
  NM_U = V1_OH,min - V2_IH,min
• The lower noise margin, NM_L is given by
  NM_L = V2_IL,max - V1_OL,max
• In order for two devices to be compatable, the noise margins must be positive.

Fanout

• FO_H = abs(I1_OH,max / I2_IH,max) where abs(.) denotes absolute value.
• FO_L = abs(I1_OL,max / I2_IL,max)
• The total fanout is given by FO = min(FO_H, FO_L)
• If the fanout has a fractional part, it should be dropped. In other words, you should always round down when calculating fanout.

Propagation Delays

• To find the total propagation delay from the input of the AND gate to the output of the OR gate, add the propagation delays of each gate.
• t_PLH = 15 ns + 9 ns + 13 ns = 37 ns (transistion shown in red)
• t_PHL = 12 ns + 10 ns + 14 ns = 36 ns (transistions shown in blue)

Tristate Outputs & Buffers

• Truth Table

E'	A	F
0	0	0
0	1	1
1	X	Z

• When the device is enabled (E' = 0), it works just like a buffer.
• When the device is not enabled (E' = 1), the output is like an open circuit.
• Buffers can be used to increase fanout because they can produce more output current than they require as an input.
• But buffers increase the propagation delay.

Transistor Labels

Bipolar Junction Transistors (BJTs)

• B = Base
• C = Collector
• E = Emitter

CMOS Transistors

• G = Gate
• D = Drain
• S = Source