Docstoc

Interfacing

Document Sample
Interfacing Powered By Docstoc
					Basic Interfacing




        ECET 2210
Overview

  • IC Drive Capability

  • LED Interfacing

  • Schmitt Triggered Inputs
  • Switch Interfacing

  • Switch Debouncing
  • Clock Generators
  • Power-On-Reset
Drive Capability

High Output
Low Output                   IOH   IIH
                   Digital               Digital




                                   IIH
                                         Digital
Drive Capability

High Output
Low Output                   IOL   IIL
                   Digital               Digital




                                   IIL
                                         Digital
LED Interfacing

 Will this work? Probably not.


                      10 - 20 mA
LED Interfacing

                        VCC


                              10 - 20 mA




     • Sinking generally provides more drive
       capability than sourcing
     • Always refer to specification sheets
Schmitt Triggered Inputs

Waveform

Thresholds

Crossings

First Crossings

Final Waveform
Schmitt Triggered Inputs

Waveform

Thresholds

Crossings

First Crossings

Final Waveform
Schmitt Triggered Inputs

Waveform

Thresholds

Crossings

First Crossings

Final Waveform
Schmitt Triggered Inputs

Waveform

Thresholds

Crossings

First Crossings

Final Waveform
Schmitt Triggered Inputs

Waveform

Thresholds

Crossings

First Crossings

Final Waveform
Switch Terminology

Types                                 Push Button

Mechanisms
Construction

                             Rocker
                 Lever                    Slide




        Rotary           Toggle
Switch Terminology

Types                • Fixed
Mechanisms           • Momentary
Construction
Switch Terminology

Types
Mechanisms
Construction
                      Single-pole Single-throw (SPST)




Poles are Pivots
                     Throws are the number of
                     possible contacts the arm can
                     touch
Switch Terminology

Types                Single-pole
                     Single-throw
Mechanisms           (SPST)

Construction         Single-pole
                     Double-throw
                     (SPDT)

                     Double-pole
                     Single-throw
                     (DPST)


                     Double-pole
                     Double-throw
                     (DPDT)
Switch Interfacing

Will this work?




                  NO - Inputs float in open position
                       Can pick up noise
                       Can damage CMOS
Switch Interfacing

Will this work?

            VCC




                  NO - Inputs float during transition
Switch Interfacing

Will this work?
                     YES - open provides high input
            VCC            closed provides low input
Switch Bounce


                   VCC




                Switch Bounce
Switch Bounce


                VCC
Switch Bounce

                                      Reset
                             VCC




 Glitch caused by bounce   Typically 50-100 ms
Debouncing a Single Throw Switch

One-Shot
Schmitt Trigger
           VCC                     Pulse > 100 ms




                  One-Shot

                               Retriggerable
                                     or
                             Non-Retriggerable?
Debouncing a Single Throw Switch

One-Shot
Schmitt Trigger
                   VCC   Similar to POR
                                   Typical values
                    33 kW




                     4.7 mF
Debouncing a Double Throw Switch

Set
                       VCC
Transition
Reset


                       VCC         Set
                                         Q

                               Reset
Debouncing a Double Throw Switch

Set
                       VCC
Transition
Reset


                       VCC         Set
                                         Q

                               Reset
Debouncing a Double Throw Switch

Set
                       VCC
Transition
Reset


                       VCC         Set
                                         Q

                               Reset
Debouncing a Double Throw Switch

Set
                       VCC
Transition
Reset


                       VCC         Set
                                         Q

                               Reset
Clock Generators

555 Timer
Schmitt Trigger
Oscillator
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
Clock Generators

555 Timer
Schmitt Trigger
Oscillator




          Chip       Freq.       R        C
       7414        0.8/RC    500     100 pF
       74LS14      0.8/RC    2k      100 pF
       74HC14      1.2/RC    10 M    100 pF
Clock Generators

555 Timer          Advantages:
                      • easily obtainable
Schmitt Trigger
                      • easy setup
Oscillator
                   Disadvantages:
                       • limited frequency
                       • not for precision applications
                       • very dependent on technology
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  1 MHz
Power-On-Reset

What is the problem with this circuit when power is applied?
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?
Power-On-Reset

The circuit:
 Power-On-Reset

 The circuit:




Vc                Vout

t+


     to                  to
Power-On-Reset

Advantages:

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

Advantages:

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

Example Solution:
Basic Interfacing

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:3
posted:8/5/2012
language:English
pages:40