What SPI "protocol"?

I just noted that a processor with hardware SPI can drive a long chain of shift registers really quick with very little CPU load. SPI may have four signals, but it's enough to use the MOSI data line and the clock line. Potentially a slave-select line. Excellent for clocking a shift register chain.

The push-pull design can work well when having bi-stable relays, or when only needing to drive one relay at a time.

The OP have probably thought that if 8x8 LED can be driven in a matrix, then it must be equally feasible to drive 64 relays the same way. But we can't drive the relays 8 times "harder" to make then stuck in the driven state for long enough time until we return to them again. Or in theory we can. With fast multiplexing we can send enough energy during the pulse that the relay will be forcibly driven. But it wouldn't be fun because of the inductive load. And we would introduce a mechanical vibration in the relay resulting in noise and wear.

But it's my guess that this is a very dead thread.