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Transcript
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 CSE 477 Interfacing 1 CMOS Inverter “Ideal” device CSE 477 Interfacing 2 Voltage/Digital Abstraction “Ideal” device: CMOS inverter CSE 477 Interfacing 3 CMOS Static Logic Logic gate: CSE 477 Interfacing 4 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 CSE 477 Interfacing 5 CMOS Interface Example This is an “open-drain” output No pullup path But we only need to sink current CSE 477 Interfacing 6 Another Open-Drain Example Question: What size should the pullup resistor be CSE 477 Interfacing 7 Data Book for CMOS CSE 477 Interfacing 8 Bipolar Logic: TTL Bipolar transistor CSE 477 Interfacing 9 Transistor as a Switch We can control Ic current by voltage on B CSE 477 Interfacing 10 TTL Logic 2-input NAND Key is the totem pole output CSE 477 Interfacing 11 TTL Voltages Squeezed towards the low end CSE 477 Interfacing 12 TTL/CMOS Transfer Characteristics CSE 477 Interfacing 13 TTL Databook Bad news: inputs source/sink substantial current CSE 477 Interfacing 14 TTL/CMOS Interfacing HCT/ACT directly compatible with TTL HC/AC is not CSE 477 Interfacing 15 CMOS/TTL Interfacing CSE 477 Interfacing 16 CMOS/TTL Interfacing CSE 477 Interfacing 17 Driving Loads with High Current We can sink some current with logic gates CSE 477 Interfacing 18 Sinking More Current Takes Real Transistors Example: CSE 477 Interfacing 19 Driving Inductive Loads Switch turns off, dI/dt induces voltage across inductor Va > Vb -> blows out the switch/transistor Protect using a diode CSE 477 Interfacing 20 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 CSE 477 Interfacing 21 Shaft encoders 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 CSE 477 IR Interfacing emitter detector pulse 22 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 CSE 477 C 2C 3C C 2C 4C 7C Interfacing 23 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 CSE 477 Interfacing 24 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 CSE 477 Interfacing 25 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 CSE 477 Interfacing 26 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 output is logic one CSE 477 Interfacing 27 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 CSE 477 Interfacing 28 A/D example - (MSB - 2) QH = 10 (VH - Vi) - 6Vi = 10VH - 16Vi 21/2 VH = 10VH - 16Vi Vi = -1/32 VH Output comparator is logic one CSE 477 Interfacing 29 A/D example - LSB 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 CSE 477 Interfacing 30
