digital design: interfacing circuits between the logic gates of same family, different families and...

Chapter 7

Interfacing Circuits between the Logic Gates Of Same Family, Different Families and Types

Lesson 2

Interconnection of an output of a gate

with input of another gate

Ch07L2-"Digital Principles and Design", Raj Kamal, Pearson Education, 2006 3

Outline

•• Magnitude and Direction of Magnitude and Direction of Source and Sink CurrentsSource and Sink Currents

• Considerations of Fan-In and Fan-Out


Current Flow

• A current flow takes place (as in any electronic circuit) howsoever small it may be

• Amount of current flowing will depend upon output at the gate and input of second gate


Current Flow

• Current always takes the easiest path (least resistance)

• Direction of this flow will depend on output logic state 1 or 0


Direction when output = 1

• The transistor at the output stage is OFF (does not conduct). Therefore, the current isource is sourced towards the input of interconnecting gate


LSTTL isource (Logic state 1)Output transistor in cut off state

Rc

Ri

VCC

VEE


Direction when output = 0

• Transistor at the output is ‘ON’(saturation state and does conduct). Therefore, the isink is sinking through it.


LSTTL isink (Logic state 0)Output transistor in saturation state

Rc

Ri

VCC VCC


Outline

• Magnitude and Direction of Source and Sink Currents

• Considerations of Fan-In and Fan-Out


74 Standard TTL • isource = 40 µA • Fan-in at 1 with respect to 74xx = 1• isink = –1600 µA • Fan-in at 0 with respect to 74xx = 1• Fan-out to 74xx = 10• Fan-out to 74LSxx = 20• IOH and IOL= 400 µA and –1600 µA


74 LSTTL • isource = 20 µA • Fan-in at 1 with respect to 74xx = 0.5• isink = –400 µA • Fan-in at 0 with respect to 74xx = 0.25• Fan-out to 74xx = 5• Fan-out to 74LSxx = 20• IOH and IOL= 400 µA and –8000 µA


40.. • isource = 1 µA • Fan-in at 1 with respect to 74xx = 0.025• isink = –40 µA • Fan-in at 0 with respect to 74xx = 0.025• Fan-out to 74xx = 0.5• Fan-out to 74LSxx = 1• IOH and IOL= 10 µA and –500 µA


NMOS• isource = 1 µA • Fan-in at 1 with respect to 74xx =

0.025• isink = –40 µA • Fan-in at 0 with respect to 74xx = 1• Fan-out to 74xx = 4• Fan-out to 74LSxx = 200• IOH and IOL= 200 µA and –2000 µA


FanFan--InIn• isource from a standard TTL twice as

large than LS TTL, and 10 times as large than from a CMOS 40... family gates

• isink in a standard TTL gate is four times than in a LS TTL gate. The reciprocal of it can be called fan-in.


FanFan--InIn• A measure of isource and isink with

respect to a standard TTL family gate at the input. Fan in is different in the states ‘1’ and in state ‘0’ for the TTL family gates.

• Same for ‘1’ and ‘0’ in CMOS family gates


Fan-in for various families of gates

• Fan-in for 74LS family at ‘1’ is 0.5 as isource (20µA) is half of the value (40µA) when standard TTL will be the input. Fan in for LS family at ‘0’ is 0.25 as isink is one fourth (400µA) from it compared to the value (1600µA) from a standard TTL gate at the input.


LSTTL Fan-in (Logic state 1 )

Output transistor in cut-off state

Rc

Ri

VCC

VEMaxi-mum

20 inputs

can connect


LSTTL Fan-in (Logic state 0 )

Output transistor in saturation state

Rc

Ri

VCC

VEMaxi-mum

20 inputs

can connect


Fan-in for various families of gates

• For CMOS 40… series gate Fan-in at ‘1’ and ‘0’ only 0.025 as isource is 40µA multiplied by 0.025 = 1µA), and isink is 1600µA multiplied by 0.025 =. 40µA.


Fan-Out

• The number of input next input stages, which can be connected to a given type of a logic gate

• Defined for an output logic gate in terms of the identical gates or in terms of standard TTL gates or in terms of LSTTL gates as the next stage(s) inputs.


Summary


• Considerations of source and sink current required at an output of a gate determine fan-in and fan-outs and determine whether interconnection feasible or not without any external voltage pull up and pull down resistances


End of Lesson 3 on Interconnection of an output of a gate

with input of another gate


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