Monopoly in Lights

Going Back and Forth on This

So, crystals came in.  Because I failed to actually check the Amazon listings very carefully, I just ordered the first crystals I saw, which were marked as 16.0 MHz.  After the order was placed and on the way, I see that the chip I’m using may not be happy with anything more than 3.0 MHz.  Oops.  Well, may as well try it anyway.

Hooked up the crystal in the circuit as demonstrated in the previously discussed page, and was pleasantly surprised to see the timing LED firing much more quickly than with the RC circuit.  Not only that, but the fact that it was firing at all was a positive sign!  Maybe it’d work with this frequency after all!  Well, let’s just connect the rest of the circuit back up and see what happens.

Well, better results, but far from acceptable.  I can see the PWM beginning to occur, but it’s not really working, so much as turning the LEDs on and off… very… slowly…  Drat.  Well, I can play around with some resistor and capacitor values to see what I can do, but that’s just not going to cut it.  I hooked up a logic analyzer and saw some interesting results.  The analyzer can only sample at 24 MHz, so I can’t expect to get a regular 16 MHz signal fully captured, but I should be able to see something.  And something I did see.  A very erratic signal, with a short burst of regular pulses that measured around 7 kHz, followed by long stretches of no pulses whatsoever, then another burst.

This means one of two things the way I figure.  Possibility A is that I’m really just not reading it well, and the signal is fine, and I’m just missing 95% of the pulses.  Possibility B is that something is still very wonky with the circuit, possibly due to the load of the other chip, and it’s just too fragile and sensitive to be reliable in the current configuration.  Considering that I also was tracing one of the higher count lines, and its pulses only seemed to occur when the bursts I did record occurred, I’m thinking option A is far more likely.

So, another timing option come and gone.  I could go ahead and order another batch of 50 crystals for a different frequency, but instead I’m going to turn to Digi-Key, who a coworker conveniently pointed out is willing to ship USPS first class mail for cheap.  I’ve got three different frequencies of actual oscillator chips on order, which I’m hoping will be far more resilient to the vagaries of my circuit that these RC and crystal configurations have been.

May 10, 2012 Posted by FoxFireX | Electronics | Comments Off on Going Back and Forth on This