SPI interface (Preliminary)

[rpalmer]

Hello everyone,

It has been quite some time since I have contributed to the hardware for CPC.

After a great deal of time dedicated to get a WEB browser (which is near to complete to handle all expected types of HTML) I have been looking at getting a simple SPI interface developed. I initially seen a ZX81 MMC circuit and thought the the CPC can use something similar for a SPI interface.

The SPI interface can be extended for a variety of functions which are developed along the lines of the SPI interface. The two that most come to mind is an SD card and/or an ethernet adapter. There is even the possibility of USB.

The attached file is a preliminary drawing using MS Excel 2010 converted to a PDF. The drawing uses "T-Juntions to indicate joins while "+" intersections are not. I am currently verifying if the circuit is okay and to date it does appear do what is expected for an SPI interface. I have also not detailed various pins as these are not connected (such as the alternate outputs of the flip-flops, while the power is 5V, but the SPI interface needs to operate at 3.3V and for this a 4050 chip can provide this for output to the SPI interface while various circuits exist to convert 3.3V to 5V.

Notes about the chips used:
1. 74192 is a 4-bit counter.
2. 74299 is a special chip which can Parallel Read/Write and also shift data left or right in or out (this makes it ideal for SPI).
3. 74175 is a Quad Flip-Flop.
4. 74138 is a 3 to 8 decoder.

I am open to any comments at this stage. I will try to get the first one developed for the classic CPC, but later models will be developed to suit the Mother X4.

rpalmer