Why are the Controls and Enables of Some of the Registers of Microprocessors SAP-1 Inverted?

Active-low logic is a fundamental concept in digital circuit design and microprocessor architecture, where a signal is active when it is low, as opposed to high. This concept has been in use for decades, particularly in memories and other digital devices. This article provides a detailed explanation of why certain registers in microprocessors like SAP-1 might use inverted controls and enables, along with the underlying principles behind active-low logic and the benefits it offers.

History and Origins of Active-Low Logic

Active-low logic has a rich history dating back to the 1970s, where even common devices like memory chips utilized this design. Pins such as ( overline{texttt{CS}} ) (Chip Select) and ( overline{texttt{OE}} ) (Output Enable) were marked with an overline, indicating their inverted state. For instance, the ( overline{texttt{CS}} ) pin would assert a low signal to enable a chip, and the ( overline{texttt{OE}} ) pin would assert a low signal to deactivate the output.

TTL Gates and Inverted Logic

The concept of inverted signals is also prevalent in Transistor-Transistor Logic (TTL) gates. TTL gates are designed to perform better at sinking a low (0) signal than driving a high (1) signal. This characteristic is due to the nature of the transistors used in these gates. For example, a 74LS series gate might exhibit an output low current ( I_{OL} ) of up to 20mA, while the output high current ( I_{OH} ) might be less than 1mA.

Consider the 74LS04 gate as an example. This gate has an output high current ( I_{OH} 0.4 text{mA} ) and an output low current ( I_{OL} 16 text{mA} ). This demonstrates the inherent advantage of sinking a low signal compared to sourcing a high signal in TTL logic.

CMOS Gate Design and Active-Low Logic

Complementary Metal-Oxide-Semiconductor (CMOS) gates, which rely on both NMOS (N-channeled Metal-Oxide-Semiconductor) and PMOS (P-channeled MOS) transistors, also benefit from active-low logic. CMOS gates use NMOS to pull an output to 0 and PMOS to pull the output to 1. Due to the lower channel resistance of NMOS transistors compared to PMOS transistors of the same geometry, CMOS gates can drive a low signal faster.

The width of the PMOS transistor can be increased to lower its channel resistance, but this adds more gate capacitance, which can slow down the signal. Therefore, for high-fanout signals like enable signals, an active-low design can provide a significant advantage.

Propagation Delay and Inverter Design

To further understand the benefits of active-low logic, it is important to look at the propagation delay of CMOS gates. A set of notes on CMOS gate design provides an excellent example. According to these notes, the propagation delay ( t_p ) of a CMOS gate is influenced by the ratio of PMOS and NMOS gate widths, denoted as ( beta ).

In an inverter, the time for a 0 to 1 transition ( t_{pLH} ) and the time for a 1 to 0 transition ( t_{pHL} ) are crucial measures. The inverter with the lowest overall propagation delay around ( beta 1.9 ) shows that ( t_{pHL} simeq t_{pLH} ). This design point ensures both transitions are balanced, leading to an optimal performance in the digital circuit.

Conclusion

In summary, the use of inverted controls and enables in microprocessors like SAP-1 is a result of long-standing design principles that optimize performance and reduce power consumption. The active-low logic ensures efficient signal transmission, particularly for high-fanout signals in CMOS and TTL circuits. Understanding these principles is crucial for designing and optimizing digital circuits and microprocessors.

By leveraging the advantages of active-low logic, designers can create more robust and efficient digital systems, making active-low logic an indispensable element in modern electronics and computing.