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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 …
The BSX Homebrew Project is an interest challenge for me, as it involves hardware and software. I’ve been concentrating on the hardware up until now, but there is a reasonable amount of software running on ROM now to assist in further hardware and software developments. If you want to cut to the quick and go straight to the source, you can find it here on my GitHub. The Monitor Program The BSX boots into the …
One of the advantages of lockdown is that spare time is often in plentiful supply. I noted a few months ago that BBC Basic for Z80 by R.T.Russell is now open source, and available under a zlib license. Inspired by the work of David Given who has included it to his CPMish project, I decided to have a crack at porting it to my BSX Breadboard Computer. I had to fettle the source files slightly …