Graham,
I’m that guy.
I chose that particular serial-to-parallel chip because it doesn’t require an external clock or timing. It syncs itself, with a maximum speed of 24KBS (19200 baud is the highest with UART). The series of bits and specifically the transition of them is the key factor. The use of 3.3V for input and 3-12V output is also great for controlling things without burdening the imp/april board. I use a separate 5VDC regulator for the TI chip output in my project.
In my application, which is working great (Christmas Light Display), I send it one burst of serial data and it sets the eight pins. I am not continually sending data. I did read that in some real-world applications some people have had to send a hex F (binary 1111) in-between the words to provide a “rest period”. I have yet to experiment with sending continuous serial bits (many frames of data).
There are other serial-to-parallel chips out there and you could even make your own using a chain of flip-flops. Nothing I have found is as simple to use as that TI chip. After Christmas, when my display comes down, I’m going to experiment with adding more TI chips to test the addressing of them.
For any imp project you have, make sure you get a breadboard so you can move wires and components around without soldering. Protect all imp/april pins as much as you can so no voltages, spikes, over current occurs. It is easy to blow out a pin. Check your wiring carefully before you power-on your project.
The imp is very addicting. Have fun with it and try new things. Keep us posted on the project(s) you work on.