
248 Analog Circuits Cookbook
switching to the high level for the final one-third, whilst for a 1 the level is
low for the first third of a bit period and high for the last two-thirds. (At
least, that is how I have described it here, in order to be consistent with
what follows - see Figure 8.22. The data sheet actually defines it the other
way round. This comes about because the address and data pins have
internal pull-ups, so that an 'on
5
(1) setting on the DIL switch pulls the
corresponding pin to ground (0), and vice versa.) The twelve transmitted
bits produced by the HT12E are preceded by a 0 as a start bit and followed
by a logic low leve ...