SCHOTTKY BIPOLAR 8224

STSTB (Status Strobe)

At the beginning of each machine cycle the aOaOA CPU is- sues status information on its data bus. Th is information tells what type of action will take place during that machine cycle. By bringing in the SYNC signal from the CPU, and gating it with an internal timing signal (<1>1A), an active low strobe can be derived that occu rs at the start of each ma- chine cycle at the earliest possible moment that status data is stable on the bus. The STSTB signal connects directly to the 8228 System Controller.

The power-on R-eset also generates STSTB, but of course, for a longer period of time. This feature allows the 8228 to be automatically reset without additional pins devoted for th is fu nction.

The READY input to the 8080A CPU has certain timing specifications such as "set-up and hold" thus, an external synchronizing flip-flop is required. The 8224 has this feature built-in. The ROYI N input presents the asynchronous "wait request" to the "0" type flip-flop. By clocking the flip-flop with <1>20, a synchronized READY signal at the correct in- put level, can be connected directly to the a080A.

The reason for requ iring an external flip-flop to synchro- nize the "wait request" rather than internally in the 8080 CPU is that due to the relatively long delays of MOS logic such an implementation would "rob" the designer of about 200ns during the time his logic is determining if a "wait" is necessary. An external bipolar circuit built into the clock generator eliminates most of this delay and has no effect on component count.

Power-On Reset and Ready Flip-Flops

A common function in a080A Microcomputer systems is the generation of an automatic system reset and start-up upon initial power-on. The 8224 has a built in feature to accomp- lish this feature.

An external RC network is connected to the R ESI N input. The slow transition of the power supply rise is sensed by an internal Schmitt Trigger. Thiscircuit converts the slow trans- ition into a clean, fast edge when its input level reaches a predetermined value. The output of the Schmitt Trigger is connected to a "0" type flip-flop that is clocked with <1>20 (an internal tim ing signal). The flip-flop is synchronously reset and an active high level that complies with the aOaOA input spec is generated. For manual switch type system Re- set circuits, an active low switch closing can be connected to the RESIN input in addition to the power-on RC net- network.

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Page 65
Image 65
Intel 8080 manual Ststb Status Strobe, Power-On Reset and Ready Flip-Flops

8080 specifications

The Intel 8085 and 8080 microprocessors were groundbreaking innovations in the world of computing, paving the way for future microprocessor development and personal computing.

The Intel 8080, introduced in 1974, was an 8-bit microprocessor that played a fundamental role in the early days of personal computing. With a 16-bit address bus, it had the capability to address 64 KB of memory. Running at clock speeds of 2 MHz, the 8080 was notable for its instruction set, which included 78 instructions and 246 opcodes. It supported a range of addressing modes including direct, indirect, and register addressing. The 8080 was compatible with a variety of peripherals and played a crucial role in the development of many early computers.

The microprocessor's architecture was based on a simple and efficient design, making it accessible for hobbyists and engineers alike. It included an 8-bit accumulator, which allowed for data manipulation and storage during processing. Additionally, the 8080 featured registers like the program counter and stack pointer, which facilitated program flow control and data management. Its ability to handle interrupts also made it suitable for multitasking applications.

The Intel 8085, introduced in 1976, was an enhancement of the 8080 microprocessor. It maintained a similar architecture but included several key improvements. Notably, the 8085 had a built-in clock oscillator, simplifying system design by eliminating the need for external clock circuitry. It also featured a 5-bit control signal for status line management, which allowed for more flexible interfacing with peripheral devices. The 8085 was capable of running at speeds of up to 3 MHz and had an extended instruction set with 74 instructions.

One of the standout features of the 8085 was its support for 5 extra instructions for stack manipulation and I/O operations, which optimized the programming process. Additionally, it supported serial communication, making it suitable for interfacing with external devices. Its 16-bit address bus retained the 64 KB memory addressing capability of its predecessor.

Both the 8080 and 8085 microprocessors laid the groundwork for more advanced microprocessors in the years that followed. They demonstrated the potential of integrated circuits in computing and influenced the design and architecture of subsequent Intel microprocessors. Their legacy endures in the way they revolutionized computing, making technology accessible to a broader audience, and their influence is still felt in the design and architecture of modern microprocessors today.