Basic Interfacing

1
Basic Interfacing
ECET 2210
2
Overview

• IC Drive Capability

• LED Interfacing

• Schmitt Triggered Inputs

• Switch Interfacing

• Switch Debouncing

• Clock Generators

• Power-On-Reset

3
Drive Capability
High Output
IOH
IIH
Low Output
IIH
4
Drive Capability
High Output
IOL
IIL
Low Output
IIL
5
LED Interfacing
Probably not.
Will this work?
10 - 20 mA

6
LED Interfacing
VCC
10 - 20 mA

• Sinking generally provides more drive
  capability than sourcing

• Always refer to specification sheets

7
Schmitt Triggered Inputs
Waveform
Thresholds
Crossings
First Crossings
Final Waveform
8
Schmitt Triggered Inputs
Waveform
Thresholds
Crossings
First Crossings
Final Waveform
9
Schmitt Triggered Inputs
Waveform
Thresholds
Crossings
First Crossings
Final Waveform
10
Schmitt Triggered Inputs
Waveform
Thresholds
Crossings
First Crossings
Final Waveform
11
Schmitt Triggered Inputs
Waveform
Thresholds
Crossings
First Crossings
Final Waveform
12
Switch Terminology
Types
Mechanisms
Construction
13
Switch Terminology
Types

• Fixed

Mechanisms

• Momentary

Construction
14
Switch Terminology
Types
Mechanisms
Construction
Single-pole Single-throw (SPST)
Poles are Pivots
Throws are the number of possible contacts the
arm can touch
15
Switch Terminology
Types
Single-pole Single-throw (SPST)
Mechanisms
Construction
Single-pole Double-throw (SPDT)
Double-pole Single-throw (DPST)
Double-pole Double-throw (DPDT)
16
Switch Interfacing
Will this work?
NO - Inputs float in open position Can pick up
noise Can damage CMOS
17
Switch Interfacing
Will this work?
VCC
NO - Inputs float during transition
18
Switch Interfacing
Will this work?
YES - open provides high input
VCC
closed provides low input
19
Switch Bounce
VCC
Switch Bounce
20
Switch Bounce
VCC
21
Switch Bounce
Reset
VCC
Glitch caused by bounce
Typically 50-100 ms
22
Debouncing a Single Throw Switch
One-Shot
Schmitt Trigger
Pulse gt 100 ms
VCC
One-Shot
Retriggerable or Non-Retriggerable?
23
Debouncing a Single Throw Switch
One-Shot
Schmitt Trigger
VCC
Similar to POR
24
Debouncing a Double Throw Switch
Set
VCC
Transition
Reset
VCC
25
Debouncing a Double Throw Switch
Set
VCC
Transition
Reset
VCC
26
Debouncing a Double Throw Switch
Set
VCC
Transition
Reset
VCC
27
Debouncing a Double Throw Switch
Set
VCC
Transition
Reset
VCC
28
Clock Generators
555 Timer
Schmitt Trigger
Oscillator
29
Clock Generators
555 Timer
The most common low-frequency clock generator
Schmitt Trigger
Advantages
Oscillator

• easily obtainable

• easy setup

• adjustable duty cycle

Disadvantages

• limited frequency ( 100 kHz)

• less stable frequency

• less accurate frequency

30
Clock Generators
555 Timer
Schmitt Trigger
Oscillator
31
Clock Generators
555 Timer
Advantages

• easily obtainable

Schmitt Trigger

• easy setup

Oscillator
Disadvantages

• limited frequency

• not for precision applications

• very dependent on technology

32
Clock Generators
555 Timer
Prepackaged clock generator requires only power
and ground connections
Schmitt Trigger
Advantages
Oscillator

• easily setup

• very precise frequency

• very stable frequency

Disadvantages

• more expensive

• primarily available ³ 1 MHz

33
Power-On-Reset
What is the problem with this circuit when power
is applied?
34
Power-On-Reset
What is the problem with this circuit when power
is applied?
Initial State?
Can we insure that the initial state will always
be the same?
35
Power-On-Reset
The circuit
36
Power-On-Reset
The circuit
37
Power-On-Reset
Advantages

• Initializes storage devices to a known state
  (i.e. counters, flip-flops, etc.)
• Provides time for power supply to stabilize

38
Power-On-Reset
Advantages

• Initializes storage devices to a known state
  (i.e. counters, flip-flops, etc.)
• Provides time for power supply to stabilize

39
Power-On-Reset
Example Solution
40
Basic Interfacing