Transcript Serial Communication

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
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CMOS Inverter
 “Ideal” device
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Voltage/Digital Abstraction
 “Ideal” device: CMOS inverter
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CMOS Static Logic
 Logic gate:
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Interfacing: Sourcing/Sinking Current
 Output Low -> Input Low:
 Output sinks current <- Input sources current
 Output High -> Input High:
 Output sources current -> Input sinks current
 The good news: CMOS inputs require very small currents
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CMOS Interface Example
 This is an “open-drain” output
 No pullup path
 But we only need to sink current
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Another Open-Drain Example
 Question: What size should the pullup resistor be
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Data Book for CMOS
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Bipolar Logic: TTL
 Bipolar transistor
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Transistor as a Switch
 We can control Ic current by voltage on B
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TTL Logic
 2-input NAND
 Key is the totem pole output
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TTL Voltages
 Squeezed towards the low end
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TTL/CMOS Transfer Characteristics
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TTL Databook
 Bad news: inputs source/sink substantial current
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TTL/CMOS Interfacing
 HCT/ACT directly compatible with TTL
 HC/AC is not
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CMOS/TTL Interfacing
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CMOS/TTL Interfacing
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Driving Loads with High Current
 We can sink some current
with logic gates
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Sinking More Current Takes Real Transistors
 Example:
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Driving Inductive Loads
 Switch turns off, dI/dt induces voltage across inductor
 Va > Vb -> blows out the switch/transistor
 Protect using a diode
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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
click!
bike wheel
baseball card
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Shaft encoders
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Instead of spokes we’ll 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
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IR
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emitter
detector
pulse
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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
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C
2C
3C
C
2C
4C
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A/D - sample
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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
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A/D - hold
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Hold state by logically controlled analog switches
Top plates disconnected from VL
Bottom plates switched from VX to VL
QH = 16 (VL - VI) = -16VI
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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
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A/D example - MSB
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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 output
is logic one
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A/D example - (MSB-1)
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QH = 12 (VH - Vi) - 4Vi
QS = QH or 21/2 VH = 12VH - 16Vi
Vi = 3/32 VH
Output of comparator is logic zero
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A/D example - (MSB - 2)
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QH = 10 (VH - Vi) - 6Vi = 10VH - 16Vi
21/2 VH = 10VH - 16Vi
Vi = -1/32 VH
Output comparator
is logic one
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A/D example - LSB
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QH = 11 (VH - Vi) - 5Vi = 11VH - 16Vi
21/2 VH = 11VH - 16Vi
Vi = 1/32
Output of comparator
is logic zero
Input sample of 21/32
gives result of 1010 or
10/16 = 20/32 or 3% error
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