chapter 7 - ic logic family
TRANSCRIPT
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BEE1213 Digital ElectronicsINTEGRATED CIRCUIT
LOGIC FAMILY
CHAPTER 7
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ICs have made digital systems more reliable byreducing the number of external interconnection fromone device to another.
ICs have reduced the amount of electrical powerneeded to perform a given function.
IC cannot handle very large currents or voltagesbecause the heat generated in such small spaceswould cause temperature to rise beyond acceptablelimits
ICs are principally used to perform low-power circuitoperations that are commonly called informationprocessing.
Introduction
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Introduction
Rapidly growth from SSI, with fewerthan 12 gates per chip; through MSI,
with 12 to 99 equivalent gates perchip
Others LSI, VLSI, ULSI and GSI
There are some things IC cannot dowhen deal with very large current
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Digital IC Terminology
VIH (min) High level input voltage. Theminimum level required for a logical 1 at aninput. Any voltage below this level will notbe accepted as a HIGH by the logic circuit
VIL (max) The maximum input voltage forlogic zero
VOH (min) The minimum voltage level at alogic circuit output in the logic 1 state underdefined load conditions
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Digital IC Terminology(cont.)
VOL (max) Low level output voltage. Themaximum voltage level at a logic circuitoutput in the logical 0 state under definedload conditions
IIH High level input current. The currentthat flows into an input when a specifiedhigh level voltage is applied to that input
IIL Low level input current. The current thatflows into an input when a specified lowlevel voltage is applied to that input
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Digital IC Terminology(cont.)
IOH High level output current
IOL Low level output current
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Exercise
Describe the input and output logic for IC7442
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Fan out
Also known as loading factor
Defined as the maximum number of
logic inputs that an output can drivereliably
A logic circuit that specify to have 10
fan out can drive 10 logic inputs
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Propagation delay
Two types of propagation delay tPLH , delay time ingoing from logical 0 to 1; tPHL delay from 1 to 0
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Noise Immunity
Stray electric and magnetic fields can induce voltages
on the connecting wires between logic circuits thisunwanted signal called noise
These cause the input signal to a logic circuit dropbelow VIH (min) or rise above VIL (max)
Noise immunity refers to the circuits ability to tolerate
noise without causing spurious changes in the output
voltage
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Noise Immunity (cont.)
Figure 8-4(a) is a diagram showingthe range of voltages that can occur at
a logic circuit output. Any voltage greater than VOH(min)
considered a logic 1
Voltages lower than VOL (max) logic0
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Noise Immunity (cont.)
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Noise Immunity (cont.)
The high state noise margin VNH isdefined as
VNH = VOH (min) VIH (min) The low state noise margin VNL is
defined as
VNL = VIL (max) VOL (max)
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Example 8-1
The I/O voltage specification for the standard TTL family are listedin the following table. Use these values to determine the following
The maximum amplitude noise spike that can be tolerated when aHIGH output is driving an input
The maximum amplitude noise spike that can be tolerated when aLOW output is driving an input
Parameter Min (V) Typical (V) Max (V)
VOH
2.4 3.4
VOL 0.2 0.4
VIH 2.0
VIL 0.8
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Power Requirements
Every IC need a certain powerrequirement to operate
This power supply is come from thevoltage supply that connected to thepin on the chip labeled VCC(TTL) orVDD(MOS)
The amount of power require by ICs isdetermined by the current that itdraws from the VCC
The actual power is ICCxVCC
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Cont.
ICC(avg) = (ICCH + ICCL)/2
PD(avg) = ICC(avg)Xvcc
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The TTL Logic Family
FIGURE 8-7 (a) Basic TTL NAND gate; (b) diode equivalent for Q1.
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TTL NOR gate
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TTL Data sheet
In 1964, Texas Instruments corporation introduced the first
line of standard TTL ICs
The 54/74 series, most widely used IC logic families
The difference between 54 and 74 series is a range oftemperatures
IC number is the same with all series produce by different
manufactures
Each manufacturer however usually used the prefix that
represent the special words Texas Instrument uses theprefix SN, National semiconductor uses DM etc
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Supply Voltage andTemperature Range
74ALS series and the 54ALS series usenominal supply voltage (VCC) of 5V, but cantolerate a supply variation of 4.5 to 5.5V.
74ALS series can operate properly inambient temperatures ranging from 0 to 70degrees C, while the 54ALS series canhandle -55 to +124 degree C.
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Voltage Levels
Input and output voltage levels can befound on the data sheet.
The min and max values shown arefor worst case conditions of powersupply, temperature and loadingconditions
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TTL Series Characteristics
We have found the type of ICs 74,74LS, 74ALS before
LS low power Schottky, ALSadvance low power Schottky
The function is same, but the
difference is on the characteristic
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TTL Data Sheet
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TTL Series Characteristics
74 series of TTL offers a wide variety ofgates and flip flops
Consist of:
a. Standard TTL, 74 series no longer beuse
b. Schottky TTL, 74S series
c. Low-power Schottky TTL, 74LS Series
d. 74AS
e. 74ALS and 74F
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Schottky TTL, 74S Series
7400 series operates using saturated switchingin which many of the transistors, whenconducting will be in saturated condition
This can causes a storage time delay ts whenthe transistors switch from ON to OFF andeffect the speed
74S series come to solve the speed problem
It accomplishes this by using a Schottky barrierdiode (SBD)
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Schottky TTL, 74S Series(cont.)
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Advances Schottky TTL, 74ASseries
74AS give moreadvance on speedswitching of TTL
ICs at much lowerconsumption
The comparison isshown in the
following table for aNAND gate in eachseries
74S 74A
SPropagationdelay
Powerdissipation
Speed-powerproduct
3ns
20mW
60
pJ
1.7ns
8mW
13.6pJ
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Advanced Low Power SchottkyTTL, 74ALS Series
Improved on bothspeed and powerdissipation
74S 74AS
Propagationdelay
Powerdissipation
Speed-powerproduct
9.5ns
2mW
19pJ
4 ns
1.2mW
4.8 pJ
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Example
Use table 8-6 to calculate the dc noise marginsfor a typical 74LS IC. How does this comparewith the standard TTL noise margins ?
Solution 74LS
VNH = VOH(min) VIH(min)
= 2.7 2.0= 0.7 V
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Example 2
Use table 8-6 to calculate the dc noisemargin for all types of 74
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TTL Loading and Fan Out
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Determining the Fan Out
How many inputs an IC output candrive
We need to know capability of theoutput
Example How many 74ALS00NAND gate inputs can be driven by a74ALS00 NAND gate output
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Example 8-6
Refer to the data sheet on the TI CDROM and determine how many
74AS20 NAND gates can be driven bythe output of another 74AS20