Bypassing the Prop Plug?

sunblock

I'm starting to feel like a nagging cold sore... I've used the recommended capacitors etc to bypass the various IC components as advised from earlier threads, and updated my documentation/drawings to boot! Note: Programmers don't (generally) do documentation--if it was hard to write it should be hard to read! (just kidding) My next question is the circuit to/from the Propeller chip and the Prop Plug. Why aren't there any 10uF bypass caps, or 0.1uF caps anywhere in the designs I can find. My guess is that they aren't necessary, but then asking can't hurt--I hope.

LoopyByteloose

A lot of 'informational schematics' have taken to omitting the by-pass capacitors and just about anything that cleans up noise or spikes.

I am not sure if Parallax has chosen to do so as well, but for the learner - all they get is the basic scheme, not the reality.

Clock Loop

The prop plug its-self has caps on it. The prop plug gets its power from its USB plug, Its only connection to anything is through ground, TX, and RX.
Your circuit should have correct capacitance in it, very close to the prop's power pins, on all sides (surface mount) and on both left and right side (DIP) need 0.1f caps non-polar.
(I use 1 uf, because I totally abuse my props and force them to draw peak current)

Cap placement in circuits usually only depends on if...... YOU the designer..... plan on drawing large amounts of elecricity.
Or if you are working around static / noisy sources. (which today is everying)

Cap placement of 0.1uf should be directly on the power pins of most microcontroller chips.
I recall that the prop liked fairly hefty direct leg caps because
WE GOTS US A BIG 8 FLIP FLOPPIN FILP FLOPPER PRO PROP-ER PROPELLER.
It needs a bit extra uumph... in the power department...

Buy like 50 of these, use them.
http://www.parallax.com/Store/Components/Capacitors/tabid/150/CategoryID/28/List/0/SortField/0/Level/a/ProductID/160/Default.aspx
I use them all over my circuit. Its a pretty common thing, if you inspect any pcb of older age, or one that has dsp/fpga on it.

Many don't think of the prop as a dsp, it kinda can be thought of as one but... it is something else all together....
It has a configurable PLL, variable clock input, self-programmable clock, 8- cores, each core can start two timers (a.k.a. spinners) that can control pins, and once they are started, the cores can choose to never come back, and the spinners will keep going forever. Other things it can do are of the more odd nature that are not even documented.
You can program 50+ prop chips all in the same time frame a single prop chip takes.

The parallel nature of parallax's propeller is a fairly undocumented area also.

I haven't really noticed if all areas that document a prop, have caps also in the documentation...
Doing this stuff for so long, i don't even see caps...heck.. I have 500 .1uf caps sitting right in front of me... and 100---220 ohm resistors...

When working with logic...
Get a HANDFULL of these...

And another 2 handfuls of these...

http://www.parallax.com/Store/Components/Resistors/tabid/149/CategoryID/27/List/0/SortField/0/Level/a/ProductID/220/Default.aspx

You had mentioned using the prop without a prop plug, and that is why I originally came to this thread.

The prop has an obex object that lets it enumerate the usb bus. Hap hazardly though, But it is a question to raise...

Could the prop run the usb bus software and emulate a serial port to windows?

Duane Degn

You'll see many form posts about the need for 0.1uF caps near each Vdd pin but they aren't shown on many official schematics. From what I've read, the bypass caps are important.

Mike Green

Digital ICs change state very quickly ... on the order of nanoseconds these days. When they change state, there can be surges of power needed briefly. Where's the power going to come from? At the speed of light, power can flow only a few inches in a nanosecond. It's not going to make it from the power supply or even across most printed circuit boards in that amount of time, so there are small capacitors placed very close to each IC or maybe they're shared between two ICs and they act as batteries for a short period of time, much longer than the couple of nanoseconds needed, supplying current to the adjacent ICs. Usually, there's a PCB electrolytic capacitor that provides the same sort of function on a larger / slower scale at the board level, usually placed where power enters the board. If the power supply is close, this might be left out.

"Could the prop run ... serial port to windows?" Well, sort of. There is a entry in the Propeller Object Exchange that does this. It's limited in its usefulness in that it takes a lot of code to do this. It only works by cheating ... it doesn't really obey some of the official rules for USB clients, so it's not possible to market a device based on it even though it works with most operating systems (Windows, Linux, MacOS). It's just that it might not work with some USB host hardware and software. It's slow. You also need a device ID which you have to buy from the USB emperor if you're going to market the device.

LoopyByteloose

I generally have found that Parallax is pretty good about showing everything in a schematic they publish. If they put it in the product, you will see it in the information.

Here is the documentation with the schematic included. It is rather silly that I have to download it to put it here so you can read it. Next time, check the product page for the documents in the Parallax store. This time is just to make sure you really can find what you want to know.

LoopyByteloose

BTW, adding capacitors to lines that are trying to send nice neat square waves will round the corners of the square waves. Eventually, the signal might even get so messed up that it becomes a glitch. By-pass caps are on the power side of things only.

Dr_Acula

Why aren't there any 10uF bypass caps, or 0.1uF caps anywhere in the designs I can find. My guess is that they aren't necessary, but then asking can't hurt--I hope.

Loopy's post #6 shows the 0.1uF bypass cap and also a 4.7uF cap. I'm not sure 10uF is set in stone as a value. Maybe it goes as low as 1uF, maybe as high as 33uF. I use 22uF a lot because I salvaged heaps from some old boards once, so then I tend to put 22uF in my designs. Maybe I (we) should write a reference range like "4.7uF to 33uF" instead of an absolute value?

I can see your point though. Looking at the demoboard, http://elmicro.com/files/parallax/propdemod_schematic.pdf I count one 0.1uF on the 3V3 rail, and given there are three chips on that board, I'd be tempted to put three 0.1uF caps. Having said that though, that is with DIP designs where the chips are further apart, and if you go back to Mike's post #5, if the chips are surface mount they are going to be a lot closer so you may well not need as many bypass caps.

Thoughts from experts would be most appreciated!

kwinn

The common practice for TTL designs was to have one 0.1uF capacitor per chip, generally placed close to one end of the chip. If there were several chips in a column then an electrolytic cap from 4.7uF to 22uF for each column was also common practice. When each board had it's own regulators (like the S100 boards) there was a large electrolytic on the input of the regulator and a smaller one on the output, and the actual values were determined by the power requirements of the board so they varied a bit more. Typical values were 470uF to 2200uF on the input and 47uF to 1000uF on the output.

With the lower power demand of current chips smaller values can probably be used for most regulators, and logic gates may only need 1 0.1uF capacitor for every 2 gates. Complex chips like micros, FPGA's, and the like will still need to have the power pins well bypassed. The best advice is to read the data sheet carefully and follow those guidelines.

kwinn

Here is a picture of a 20 channel 32 bit scaler/counter board that shows the capacitors. The on board regulator input came from a 55,000 uF capacitor via about 12 inches of wire so no input capacitor was required, but there were two 220 uF capacitors on the output.