Zilog Z80A
The Zilog Z80A is a processor first manufactured by the MOSTEK Company in approximately July 1976 as an improvement over the Intel 8080, which had been the only choice for computer designers. Originally designed for the calculator industry, the processor ended up in a variety of applications ranging from home computers to military. The team that designed the Intel 8080, designed the Z80, and consequently there are many similarities in their architecture. There are similarities in the register set such as overlapping fetch and execute cycles, and the ability to combine 8-bit registers to form 16-bit precision, thus enabling programmers to write code without requiring new training.
The Z80 was also compatible with the Intel 8080 microcode so that it could run the CP/M Operating System. It had many improvements over the Intel 8080, one notable one being with the naming scheme of the assembly language mnemonics, which were more systematic and straightforward to understand and learn.
SOC or Z80A
In comparison with modern ARM based system-on-chip (SOC) packages, this 8-bit Z80A is far simpler and easier to work with, and consequently students design and build their first computer based on this processor. You will learn far more electronics with the Z80 than a SOC package, which is just a black box. The ARM11 core that they use is usually a highly protected corporate secret, and you will not find any detailed information about its architecture apart from the bare minimum that one would need to program its GPIO pins. Often manufacturers will not even provide any architectural information about the SOC processor except the bare minimum; therefore, they are just black boxes with GPIO pins. One also ends up being dependent upon the manufacturer to provide the drivers to enable its internal functionality, hence, SOC processors cost additional money and require license agreements for anyone who wants to use them.
Currently there are countless computer projects based around the Z80 processor, and I just loved the ZX97, and ZX96 projects, which are improvements over the Sinclair ZX81, and worth following. You can also build your own ZX80 as well, and the plans are all available on the Internet, and you just need to get the parts to build it. However, it is much more exciting to design and build your own computer, even if it is a simple design consisting of single RAM and ROM chips.
After seeing Steve Wozniak’s computer boards, which he had designed and built, I built my first Zilog Z80 computer in 1994, which was constructed on three joined pieces of breadboards.
Z80A Interfacing
An 8-bit Z80 CPU in an embedded design, and it is still the most cost effective option for almost any kind of automation project. Not only is it a cheaper processor but it is also cheaper to interface circuits to it. Whilst a 32-bit processor will require around four ROM and RAM chips per word, the Z80 will work fine with just a single ROM and RAM chip. It is efficient because it handles a byte of data in an 8-bit bus. Building a computer based on the Z80 can be a joy because of the smaller 8-bit bus, and I have frequently built a complete system on a breadboard without any difficulties. The great thing about this processor is that it supports 16-bit memory addressing and 8-bit I/O addressing. As a designer, you have the option to choose whether you want to decode the entire 16-bit bus or just the higher half of the bus. This flexibility helps to reduce the component count and simplifies the computer design.
Found on many modern microcontrollers today, the Z80 core continues to be one of the most efficient circuits for embedded applications. The core is essentially the same as it was when introduced in the 70s. However, modern CMOS implementations can allow it to operate at speeds of 20 MHz and higher. With the price of modern memory dropping, it is possible to interface this processor with fast DRAM chips to achieve good performance.