14.1 14.2 Signal Types • Recall even digital signals are just voltages … • Analog signal – Continuous time signal where each voltage level has a unique meaning • Digital signal Unit 14 – Continuous signal where voltage levels are mapped into 2 ranges meaning 0 or 1 Noise Margins, Interfacing, and volts volts 1 Tri-States 1 Threshold 0 0 0 time time Analog Digital 14.3 14.4 Signals and Meaning Analog Digital 5.0 V 5.0 V Logic 1 2.0 V Illegal Threshold Range 0.8 V NOISE MARGINS, LEVEL SHIFTERS, Logic 0 & DRIVE STRENGTH 0.0 V 0.0 V Each voltage value Each voltage maps to ‘0’ or ‘1’ has unique meaning (There is a small illegal range where meaning is undefined since threshold can vary based on temperature, small variations in manufacturing, etc.)
14.5 14.6 A Motivating Example The Digital Abstraction • Digital is a nice abstraction of voltage and current – Lets us just think 'on' or 'off' but not really worry about the voltages and Example 1 Example 2 currents underneath • You connect an output port to an LED • You buy two digital chips (say a • ______________ (light emitting diode) and connect microprocessor and GPS reader • Not all 1's and 0's are created equal everything correctly . The light should • You correctly wire them together turn on when you set your output bit to – A '1' can be any 'HIGH' voltage (maybe in the range ___________) and write software to turn 'on' a a high voltage (logic '1'). pin on the microprocessor to a '1' – A '0' can be any 'LOW' voltage (maybe in the range ___________) • When you turn the system on the LED to enable the GPS reader – So 3V and 5V both mean ________ but they aren't equal does not glow. You measure the voltage at the gate output with a • When the software runs the GPS • Similarly certain outputs of a chip may connect to other devices voltmeter and find it is not 5V but unit does not turn on. Why? that require more _________ than the output can _________ 1.8V? Why isn't it a logic 1? • Different circuit implementation – Think of connecting a ______________ to your garden spigot • The ______________ output ability techniques use different voltage from the output port is not ____ – Or even worse your garden hose to a fire _________... would shred it ______ to indicate _________ enough to adequately ______ the LED and may be _____________ – In the same way, inputs and outputs of different devices must be matched to which then drags the voltage _______. the _____________________ of what they connect to Lesson To Be Learned: Not all 1's or 0's are created equal! 14.7 14.8 Digital Voltage Noise Margins Digital Voltage Noise Margins • Consider one digital gate feeding another OH = Output High OL = Output Low • Consider the output of one digital circuit feeding the input of another IH = Input High – Assume the devices are from different vendors (families of devices) IL = Input Low NM = Noise Margin • There may be different ___________ and requirements of the two devices – Example: The output may produce 3V to mean logic '1' while the next device's ______ Range ______ Range Interpretation Interpretation input requires 5V to be used as logic '1' 5.0 V 5.0 V • Analogy 1: Grades. Suppose the cutoff for an A is 90% (i.e. __________ input) – If you get a 91% (i.e. output result)… _______! NM H = Logic 1 Logic 1 V OH As long as – If you get an 89%…(__________ for this class! But ______ from the cutoff's ______ _________ and perspective.) Possible Output _________ we are • Analogy 2: Tickets. Suppose there are 100 available tickets to an event (i.e. in good shape… Required Input V IH input limit) Electromagnetic – If you are the 99 th person (i.e. output result)… ________! interference & power Illegal Illegal spikes can cause – If you are the 101 st person… __________! this to break down V IL Logic 0 Logic 0 V OL NM L = 0.0 V 0.0 V Input Output _______
14.9 14.10 Class Activity Analogy • Consider a sprinkler system…what will happen if you add 100 • Do an internet search for "74LS00 datasheet" new sprinklers to your backyard? (this is a chip w/ some 2-input NAND gates) • Pressure (voltage) will go ______________ and ___________ and try to find any PDF and open it water (current) flow coming out of each • Skim the PDF and try to find: – VOH, VIH, VOL, VIL 14.11 14.12 Current Limitations Example • When a circuit outputs a 'HIGH' ('1') it can only supply ( __________) so • Consider the example where device A's output much current (think of your garden hose spigot) = I OH connects to device B's input • When a circuit outputs a 'LOW' ('0') it can only suck up ( __________) so much current = I OL – Are the voltage requirements compatible? • When a circuit receives a 'HIGH' signal on the input side it may need a – How many device B inputs can a single device A output certain amount of current to recognize the input as 'HIGH' = I IH drive? • When a circuit receives a 'LOW' signal on the input side it may need a certain amount of current to recognize the input as 'LOW' = I IL • Always use worst case of ______________ output drive capability Dev. VOH VIH VOL VIL IOH IIH IOL IIL I IH I IL A 3.4V 3.3V 0.5V 1.0V -4 mA -1 mA 10 mA 2 mA B 3.2V 3.0V 0.6V 0.7V -2 mA -1 mA 6 mA 2 mA 1 0 Voltage requirement are _____________ Dev. A's output can drive 4 Dev. B inputs Dev. A VOH ___ Dev. B VIH When outputting '1': I OH I OL AND - (Dev. A IOH / Dev. B IIH) = (________) = ___ Dev. A VOL ___ Dev. B. VIL When outputting '0': - (Dev. A IOL / Dev. B IIL) = (________) = ___ Drive capability = ___________________
14.13 14.14 Consideration All In the Family • If we attach too many gates to one output it • There are many families of circuit devices that talk different may not be enough to drive those gates language (Each has a different VOH, VIH, VOL, VIL, IOL, IIL, etc.) • Need to make sure the current • Examples: requirements and capabilities match – _____________ • Let's say we connect one of the NAND gates – _____________ on the 74LS00 chip to an input of N other – _____________ If I OH or I OL is too low we can NAND gates… split the loads by place • Must make sure if you interface two different devices that they intermediate buffers • Can it produce/suck up the required are ________________ (i.e. VOH of device A is greater than VIH current… of device B) or use a buffer/amplifier/level shifter circuit to help • …if N = 6? them talk to each other – http://www.ti.com/lit/ds/symlink/cd4504b-ep.pdf • …if N = 12? VOH=2.2V VIH=3.5V A B 14.15 14.16 Arduino Limits Another Example • Now consider a speaker system where the power and signal • Arduino outputs can sink (suck up) and source (produce) are provide together around a maximum of 20 mA on a pin – Given our Arduino use 5V = Vcc and its current limitations per pin, how – http://www.atmel.com/Images/Atmel-8271-8-bit-AVR- much power can we supply to the speaker? Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328- 328P_datasheet.pdf – 5V * _____________ = ____________ • Do an internet search for "Standard Servo Motor Datasheet" – You need an _________________… and find the maximum current it may need Power & Signal • It doesn't seem like the Arduino would be together able to drive the servo motor. How is it working? – Remember the 3-pin interface: R = Power, B = Ground, W = Signal – The signal is _____________ from the power – The power source is used to amplify the signal
14.17 14.18 Typical Logic Gate Hot Water = Logic 1 • Gates can output two values: 0 & 1 Cold Water = Logic 0 – Logic ‘1’ (Vdd = 3V or 5V), or Logic ‘0’ (Vss = GND) (Strapped together so always one type – But they are ALWAYS outputting something!!! of water coming out) • Analogy: a sink faucet – 2 possibilities: Hot (‘1’) or Cold (‘0’) • In a real circuit, inputs cause EITHER a pathway from output to VDD OR VSS +3V +3V Inputs Vdd PMOS PMOS TRI-STATE GATES Inputs Inputs Output Output NMOS NMOS Vss 14.19 14.20 Output Connections Tri-State Buffers +3V • Can we connect the output of two logic gates together? • Normal digital gates can output two • _______! Possible _______________ (static, low-resistance values: 0 & 1 PMOS pathway from Vdd to GND) 1. Logic 0 = 0 volts Inputs Output • We call this situation “_________________” 2. Logic 1 = 5 volts Z (high • Tristate buffers can output a third Inputs impedance) Vdd NMOS value: Src 1 Src 1 3. ____ = __________________ = "Floating" (no connection to any voltage source…infinite resistance) Vss Src 2 • Analogy: a sink faucet Inputs Vdd – 3 possibilities: 1.) Hot water, Hot Water = Logic 1 Src 2 2.) Cold water, Src 3 Cold Water = Logic 0 3.) _____ water Vss ___ Water = Z (High-Impedance)
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