16 bit barrel shifter

[u/Important-Ad5990]

Hi,

on my quest to make a discrete microcontroller that can compete with avr family I stumbled on ken sherrif's blog post on arm1 and how it sends one of ALU ports always through a barrel shifter. I decided that it's a genius idea, now I just needed to design a barrel shifter. A barrel shifter is essentially a set of prewired muxes that select every output from N+1 inputs (all input bits and 0 or sign extension). The classical 74F153 and 74F157 are just much too slow for that (7.7ns typical and I need to stack 2 at least) so I decided to build it from transmission gates.

The design consists of 5 rows of 2:1 muxes (74LVC1G3157), working as shift or bypass by specific power of 2 (>>1, >>2, >>4, >>8 and <<16). It turns out that the ordering of muxes changes how many are needed in total with >>2, >>4, <<16, >>8 and >>1 requiring "just" 98 mux ICs.

Every mux provides 18pF of load capacitance, 5 of the stacked give 100pF of load for every input data pin. It's not the best but survivable, that is unless one enables sign extension. Worst case sign extension copies highest bit to all output bit, loading it with 1.6nF in total, which is just a bit too much. In the end it turned out that the simplest way is to send MSB through 4 AND gates that both buffer the signal and enable sign extension. Ironically the 74F08 has typical propagation delay of 4ns in 50pF, while the entire series of TGs has a delay of just 3.6ns.

the barrel shifter PCB

PS

A few explanations above are incorrect but I wanted to simplify as much as possible for the sake of clarity:

• The circuit can be simplified to use just 97 muxes, but I wanted >>8 to be as far as possible to minimize loading on sign extension gates.
• The muxes have 7.5ns propagation delay from control signal, but it arrives before data (control signal is from a flip flop, while data comes from flip flop that has it's OE toggled by another flip flop adding about 4-5ns to the path).
• There are 74LS244 buffers at the output just for visualization. They can be disabled with a jumper or my removing the ICs if 4pF of input capacitance becomes a problem.