Designing the RhoCoCo Retro Home COlor COmputer hardware

Less is more, less complexity, more fun is the driving force behind the Rhococo Retro Home COlor Computer. It will be a remake home-computer inspired on the Sinclair spectrum, but not based on a Z80, but on an ARM7TDMI based simple micro-controller, (probably an STR710FZ2) running at 66MHz with 256 + 16 KB internal fast Flash and with 64K internal fast RAM and a separate external 32K VRAM. The idea behind it is no less than bringing back the home computer of decades past.

It will be capable of 320x200 VGA full high resolution graphics 16 colour display (Mode "D" timings, 70Hz, Horizontal Sync = 31.778 KHz, pixel clock 25.175 MHz).

To be able to keep the design simple, but still able to be drive a VGA monitor, I chose to support only the lowest VGA resolution of 320 x 200 with 16 colours. It means we only need to have, (and manipulate) a 256000 bit (32K) frame buffer. The display logic will be designed to read out four bytes (a 32 bit "word") in the time assigned to eight consecutive pixels, three bytes will be shifted to the Red, Green an Blue colour inputs, and one to the intensity input, giving sixteen colours per pixel. Preferably the "glue logic" for it will consist of standard logic, but if necessary I will consider a small FPGA. (Benadski: If a FPGA is used, can you choose your own palette in software? It would be a cool feature!)

the Rhococo will support a simple SD-Card interface as storage, and will use a keyboard similar to the ZX81+34, but with more keys,and a more conventional keyboard layout.

For sound the ARM chip will offer a PWM output.

It will be a "turn on and use" system, meaning that the "OS", and a programming language will permanently reside in ROM, ready the moment you turn the Rhococo on.