PLL protection question

tk3000

Hi folks, I've experienced some PLL burnout issues similar to the ones seen here:

http://forums.parallax.com/forums/default.aspx?f=25&m=179308&g=181717#m181717

I've made my first PCB and I would like to use it for an upcoming workshop I'm going to be giving. However, I am concerned about others in the workshop experiencing the issue to. The application uses video (hence PLL), so once we've lost that functionality we're soaked.

I'm not an electrical engineer, but here's what seems to happen. Introducing a short into the circuit surely seems to fry the chip, which I have certainly done. Maybe I'm crazy but I'm getting superstitious about plugging and unplugging the DC power and plugging and unpluggin the Prop Plug. After the PLL is fried, there's a big power draw and the Propeller and the voltage regulator get extremely hot.

What I would love to know is if there are some simple elements i can introduce to my circuit--it's a simple circuit to help prevent this error from happening. For instance, what values for decoupling capacitors are the best--ceramic or aluminum? Would adding diodes or some simple circuit to prevent electrical backflow in a short-circuit situation help? Is one brand of voltage regulator better than another? Should there be a pullup resistor attached to RESn (I've noticed that plugging and unplugging the Plug seems to reset the prop). Any tips on circuit design or parts to help prevent this issues would be greatly appreciated.

thanks much!

Paul Baker

Our experience with PLL failure centers around spikes on the power, the power lines of the current Propeller do not have the ESD protection of the I/O pins so they are more suceptible to surges and seem to target the PLL of the oscillator (the likely path of least resistance). If you are not doing motor switching or other inductive loads such as relays you shouldn't have an issue (I have deliberately mistreated Propellers and never had one die on me). Be sure you have bypass caps and adequate caps on the input, this will deal with any glitches on the plug/unplug of the chip. If you are switching inductive loads use a snubber diode to provide a safe path for the collapsed field.

For bypass caps I like to use 0.1-1uF tantalums, for on board v-reg I follow the datasheet's recomendation, with a regulated supply I use a 100uF electrolytic next to the jack.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

tk3000

Great, thanks Paul.