multiplexing several channels

Question

I'm designing a 6-channel amplifier which requires one LPF, one HPF, and one gain stage for each channel. I use 4*1 multiplexers for LPF and HPF, and 8*1 multiplexer for gain to control these multiplexers. So I have 7 selection pins for each channel. Now for 6 channels, I need 42 selection pins.

This is simply too much for me. My controller has at most 20 digital outputs. So I think I might need to use something like multiplexing. But the problem is that I can't do sweeping because none of these filters can be left floating. I can't even think of any solution to google for answers. Can anyone suggest a way?

Answer 1

Fewer than 20 IOs can achieve this if you use latches like the 74HC573: -

If you parallel several of these (say) 16 to give up to 128 outputs you need 8 IOs to drive D0 to D7 (common to all) and 4 IO decoded to produce an extra 16 outputs to drive the individual LE lines.

Here's a design of a parallel IO card that is similar: -

It operates slightly different to how I described but, in principal produces a multitude of IO lines all controlled from a few IO lines. The clever bit about the above design is that it also uses an extra 573 for the LE lines on the 573s used to generate the wide IO range.