In Part 4 of this series I implemented a proper clock signal using a crystal and logic, in readiness for adding the TMS9918 video chip into the circuit. This provides a 5.37Mhz clock signal for the Z80, along with a 10.738Mhz signal for the video chip, and boosted the Z80 clock speed over fivefold from the original ~1Mhz the 555-timer based clock. However, I discovered this caused comms instability with the software UART I’d implemented …
I decided quite early on in this project that I’d try to use interesting and contemporary technology to build the BSX, and make the BSX as simple to build on a breadboard as possible. The GAL is a perfect fit for all those criteria; it’s a technology I’ve not used before, it will cut the logic chip count down somewhat, and is a component that’s been around since the 1970s. What is a GAL A …
The computer is now a fully operational Z80 board, with ROM, RAM and I/O. All I/O is through a serial USB connection to a terminal package (such as PuTTy), and there is some software on ROM, my Z80 monitor program, and BBC Basic for Z80. It is still running on a slow (~1Mhz) clock provided by the clock circuit devised in Part 1. The next step is to implement a faster and more accurate clock …
