COE202lesson10 - Fanout Limitations If the output of some...

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Fanout Limitations If the output of some gate A is connected to the input of another gate B, gate A is said to be driving gate, while gate B is said to be the load gate. As the Figure shows, a driver gate may have more than one load gate. ™There is a limit to the number of gate inputs that a single output can drive. The fanout of a gate is the largest number of gate inputs this gate can drive. ™For TTL, the fanout limit is based on CURRENT. ƒ A TTL output can supply a maximum current IOL = 16 mA (milliamps) ƒ A TTL input requires a current of IIL =1.6mA. ƒ Thus, the fanout for TTL is 16mA/1.6 mA = 10 loads. ™ For CMOS, the limit is based on SPEED/propagation delay. A CMOS input resemble s a capacitive load (≈10 pf - picofarads). ƒ The more inputs tied to a single output, the higher the capacitive load. The HIGHER the capacitive load, the SLOWER the propagation delay. ƒ Typically, it is advisable to avoid loads much higher than about 8 loads.
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  • Fall '13
  • Sait
  • Gate, single output gate

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