Abstract digital values are fine but...

1
Interfacing

• Abstract digital values are fine but...
• We have to deal with the realities of voltage and
  current
• e. g.
• Technology CMOS vs. Bipolar
• Voltage level 5v vs. 3.3v vs. 2.5v
• Current sink/source

2
CMOS Inverter

• Ideal device

3
Voltage/Digital Abstraction

• Ideal device CMOS inverter

4
CMOS Static Logic

• Logic gate

5
Interfacing Sourcing/Sinking Current

• Output Low -gt Input Low
• Output sinks current lt- Input sources current
• Output High -gt Input High
• Output sources current -gt Input sinks current
• The good news CMOS inputs require very small
  currents

6
CMOS Interface Example

• This is an open-drain output
• No pullup path
• But we only need to sink current

7
Another Open-Drain Example

• Question What size should the pullup resistor be

8
Data Book for CMOS
9
Bipolar Logic TTL

• Bipolar transistor

10
Transistor as a Switch

• We can control Ic current by voltage on B

11
TTL Logic

• 2-input NAND
• Key is the totem pole output

12
TTL Voltages

• Squeezed towards the low end

13
TTL/CMOS Transfer Characteristics
14
TTL Databook

• Bad news inputs source/sink substantial current

15
TTL/CMOS Interfacing

• HCT/ACT directly compatible with TTL
• HC/AC is not

16
CMOS/TTL Interfacing
17
CMOS/TTL Interfacing
18
Driving Loads with High Current

• We can sink some currentwith logic gates

19
Sinking More Current Takes Real Transistors

• Example

20
Driving Inductive Loads

• Switch turns off, dI/dt induces voltage across
  inductor
• Va gt Vb -gt blows out the switch/transistor
• Protect using a diode

21
Shaft encoders

• Need to determine the wheel velocity
• Use sensor to detect wheel moving
• Determine speed of a bicycle
• attach baseball card so it pokes through spokes
• we know number of spokes
• count clicks per unit time to get velocity
• Baseball card sensor is a shaft encoder

bike wheel
baseball card
22
Shaft encoders

• Instead of spokes well use black and white
  segments
• Black segments absorb infrared light, white
  reflects
• Count pulses instead of clicks
• We could use a light source and
  transparent/opaque segments

wheel
IR
emitter detector
pulse
23
Analog to digital conversion

• Use charge-redistribution technique
• no sample and hold circuitry needed
• even with perfect circuits quantization error
  occurs
• Basic capacitors
• sum parallel capacitance

C
C
2C
C
3C
7C
24
A/D - sample

• During the sample time the top plate of all caps
  switched to VL
• Bottom plate set to unknown analog input VX
• Largest cap. corresponds to MSB
• Q CV
• QS 16 (VX - VL) 16VX

25
A/D - hold

• Hold state by logically controlled analog
  switches
• Top plates disconnected from VL
• Bottom plates switched from VX to VL
• QH 16 (VL - VI) -16VI

26
A/D - approximation

• Conservation of charge QS QH so VI -VX
• 16 VX -16 VI
• Each cap. switched from VL to VH
• Output of comparator determines bottom plate
  voltage of cap
• 1 remain connected to VH
• 0 return to VL

27
A/D example - MSB

• Suppose VX 21/32 VH , VI -VX -21/32 VH
• QS 16VX 16 (21/32) VH 21/2 VH
• QH 8 (VH - VI) 8 (VL - VI) 8VH - 16VI
• QS QH or 21/2 VH 8 VH - 16VI
• VI -5/32VH
• Comparator outputis logic one

28
A/D example - (MSB-1)

• QH 12 (VH - Vi) - 4Vi
• QS QH or 21/2 VH 12VH - 16Vi
• Vi 3/32 VH
• Output of comparator is logic zero

29
A/D example - (MSB - 2)

• QH 10 (VH - Vi) - 6Vi 10VH - 16Vi
• 21/2 VH 10VH - 16Vi
• Vi -1/32 VH
• Output comparatoris logic one

30
A/D example - LSB

• QH 11 (VH - Vi) - 5Vi 11VH - 16Vi
• 21/2 VH 11VH - 16Vi
• Vi 1/32
• Output of comparatoris logic zero

Input sample of 21/32 gives result of 1010 or
10/16 20/32 or 3 error