Key Concepts in Microcomputer Principles

1. Bus Contention and Load

  • Bus Contention: Occurs when multiple devices attempt to send signals on the same bus at the same time.

    • For TTL Logic Circuits: Simultaneous output by two or more devices may create an unstable state on the bus, potentially damaging hardware.
    • For Open Collector (OC) Output: Uses wired logic to avoid damage; however, information from one device may be lost due to overlap.
    • Solution: Utilize tri-state gates and control their logical states to avoid conflicts. When one gate is in a high-impedance state (Z), contention is prevented.

  • Bus Load:

    • DC Load: The CPU must supply sufficient current for each chip on the bus.
    • Output Current (IOL and IOH): Defines the maximum current when a gate outputs high (IOH) or low (IOL).
    • Input Current (IIL and IIH): The current absorbed when a gate inputs high (IIH) or low (IIL).
    • Calculation: Ensure that the driver gate’s output current (IOH and IOL) exceeds the total input current of all load gates.
    • Fan-out: Indicates the number of identical load gates a single driver gate can support.

2. Comparison of Different Logic Series

  • TTL (Transistor-Transistor Logic) vs. CMOS (Complementary Metal-Oxide-Semiconductor)

    • TTL Series: Includes series like 74, 74LS, 74ALS; uses bipolar transistor technology.
    • CMOS Series: Series like 74HC, 74HCT; manufactured with CMOS technology.

  • Parameter Comparison:

    • Input Current (IIH/IIL): TTL generally has higher input current than CMOS.
    • Output Current (IOH/IOL): TTL offers higher current but also higher power consumption.
    • Voltage Threshold (VIH/VIL, VOH/VOL): CMOS supports a broader voltage range.
    • Supply Voltage (Vcc): CMOS can operate over a wider voltage range.
    • Propagation Delay (tpd): High-speed TTL like 74ALS and some CMOS (like 74HC) offer fast transmission.
    • Fan-out (NO): CMOS usually has higher fan-out, supporting more loads.
    • Power Consumption (Pd): CMOS is generally more power-efficient.