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SN74ABT651: DESIGNING WITH LOGIC

zhang ping
Genius 3835 points
Part Number: SN74ABT651

Tool/software:

，I have‘nt  part number，this question is for TI  all logic ；For the control pins of logic gate devices and other devices, if they are at a fixed level, it is recommended to directly connect them to the power supply and ground, or to connect them in series with a resistor and then connect them to the power supply and ground. What is the required number of resistors in series; In the article "Design with Logic" from 1997, it was recommended to directly connect power and ground to all devices except for a few special structural devices. Is there an updated version of this article, and are there any other documents explaining it?

  • 0
    Guru 316690 points

    Designing With Logic says on p. 7:

    Devices with multiple-emitter inputs (SN74 and SN74S series) are exceptions. Since no voltage greater than 5.5 V should be applied to the inputs (because if exceeded, the base-emitter junction at the inputs breaks down), the inputs of these devices must be connected to the supply voltage VCC via series resistor RS (see Figure 11).

    Fairchild's Designing with TTL has a more detailed explanation:

    Inputs are usually tied to VCC through a 1 kΩ to 5 kΩ resistor, since tying them to ground means supplying the IIL current instead of the IIH current. IIL is several orders of magnitude greater than IIH. The resistor is recommended to protect the input against VCC voltage surges and to protect the system against the possibility of the input shorting directly to ground. A single 1k resistor can handle up to 10 inputs.

    There are no other logic families with multiple-emitter inputs; this recommendation has not changed. (And even with TTL and Schottky families, the resistor is necessary only when overvoltage can actually happen. If you connect the input to the same VCC and have a decoupling capacitor, it is not possible for the input to surge above VCC.)