December 28th, 2005


At long last...

Finally I have gotten the shift register train to work -- after more than ten days of fiddling with it.
I connected the powerlines (both positive voltage and ground) on both ends of the little strip of development board that holds the shift registers and the AND-IC -- if power fluctuations towards the end of the 'train' were the problem, then that should have fixed it. But it didn't.

Then I disconnected all the connections from the third shift register. The shift registers share a common Output Enable, Clock and Strobe lines, all driven from a single pin from the PIC. If I disconnected the Clock and Strobe lines, but left the IC in the socket, the other two shift registers worked as intended!
I reconnected the Strobe line, and still all was well. Then I reconnected the Clock line, and things were defective again.

Apparently the voltages and/or the current on the Clock line was insufficient to drive three shift registers. I do drive that line straight off a pin of the PIC, but that was not a problem in the MACH F display.
So I changed the circuit: the pin from the PIC would drive a transistor, with the collector connected to the positive voltage 'power rail' (which sounds impressive, but it is really just a bunch of wire connections tied to the positive voltage from the wallwart adapter) -- this should ensure a correct voltage and as much current as the ICs want/need.

Alas, that did not work. I have absolutely no idea why.

So, I did what any engineer would have done: I worked around the problem. I still have a few pins left, so I connected one of the pins to the Clock pin of the third shift register. Instead of pulling one pin high or low, I now pull two pins high or low. And it works.

I'm not sure why -- but it works. And after ten days of frustration (sometimes I wonder why I consider this a hobby...), I'm not going to pick any nits regarding the solution.

Les fleur

This morning, when the alarm clock went off by playing the radio, I heard a song that I vaguely recognised. What with the chorus, it sounded like something from a rock musical.
I asked klik whether she recognised it -- perhaps it was from Jesus Christ Superstar, or Hair? But she mumbled something incoherently and stumbled towards the bathroom, leaving me to ponder...

This afternoon, my colleague B. played some music over the speakers. And what do you know: the same song came up. So I asked him where it was from, but he didn't know either. He did have the title and the artist (a tremendous advantage of MP3s, IMO).

Turns out that it was Les Fleur by Minnie Ripperton. And no, it was not from any musical. But it did get used in a commercial.
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