Board 2 : Register set

A project log for Discrete YASEP

a 16-bits YASEP computer (mostly) made of DIP/SOIC chips like in the 70s and 80s... with 2010's twists!

yann-guidon-ygdesYann Guidon / YGDES 10/20/2015 at 19:590 Comments

(Obsolete but interesting anyway)

The second board is the simplest and the illustration of many of the principles of this project.

Here is a more-or-less 1:1 scale layout :

It's "simply" a bunch (20) of 74HC573 8-bits D-FlipFlops.

In the YASEP architecture, registers R1 to R5 are "simple", just like in most RISC designs. They have no side effect : they can be read and overwritten like any memory cell.

They are "materialised" by these 8-bits SOIC chips and provide a physical sense of where data reside.

Each register is 16-bits wide so 2 SOIC chips are required per read bus.

There are 2 read busses (SI4 and SND) so the chips are duplicated (the data are stored redundantly). This is actually a compromise to reduce chip count. Each SOIC chip will drive its read bus only when selected, its output remains in high impedance otherwise. Other methods of 2reads-1write would instead use 1 DFF chip and 2 multiplexing chip (either a 8->1 MUX or a 74HC245).

A normal MUX is the preferred method for high-speed designs but the YASEP's register set is not homogenous and it would make the board too complex.

Three 74HC138 select one of the 5 registers for read and write. Outputs 0, 6 and 7 are left open on this board but other boards (memory) use them.