General Electronics Questions
SRLM
Posts: 5,045
Well, I've been collecting some questions and browsing hasn't yielded any answers, so hopefully some helpful people will drop a yes or no.
1. For SPI devices, is it always safe to connect Din and Dout to the same I/O pin?
2. For SPI devices, is it okay to tie the CS pin high (as long as there is only one chip on the bus), or does this vary by device (the ADC below says CS must change)?
3. Is it safe/okay to load up the power rails with capacitors? Is it possible to have to many?
4. Do resistors on digital pins (such as between a propeller I/O and an external 3.3v device) create a maximum speed situation (something about reflections)?
5. I have three of these A/D converters, two of which are hooked up but without analog inputs yet. I noticed when running the code that it seems one settles at the high values (~4095), while the other settles at the low value (~0). Is this normal? I'd assume both chips would have the same "no connection" value.
6. I've been sizing wires based on AWG tables and current requirements(here's one), but when I looked at a RC airplane setup with currents up to 50 amps, the wires were smaller than I expected (12 or 14 gauge). Is an AWG table a good way to size wires?
Thanks for any answers provided.
1. For SPI devices, is it always safe to connect Din and Dout to the same I/O pin?
2. For SPI devices, is it okay to tie the CS pin high (as long as there is only one chip on the bus), or does this vary by device (the ADC below says CS must change)?
3. Is it safe/okay to load up the power rails with capacitors? Is it possible to have to many?
4. Do resistors on digital pins (such as between a propeller I/O and an external 3.3v device) create a maximum speed situation (something about reflections)?
5. I have three of these A/D converters, two of which are hooked up but without analog inputs yet. I noticed when running the code that it seems one settles at the high values (~4095), while the other settles at the low value (~0). Is this normal? I'd assume both chips would have the same "no connection" value.
6. I've been sizing wires based on AWG tables and current requirements(here's one), but when I looked at a RC airplane setup with currents up to 50 amps, the wires were smaller than I expected (12 or 14 gauge). Is an AWG table a good way to size wires?
Thanks for any answers provided.
Comments
(2) Beats me
(3) You can have too (not to) many if the device collapses from the extra weight
(4) Nothing about reflections, but if there is capacitance at the far end the resistance can cause a slowdown. In general, resistance in a digital line, up to several thousand ohms, will have no noticeable effect except for the beneficial one of protecting the devices from each other.
(5) Without looking at the spec sheet, which is not available through your link, I'd guess it's not surprising. Check them out by giving them (temporarily) something to look at. If it bothers you, ground the unused inputs.
(6) The AWG table is a good way to size wires if you're putting hundred-foot wire runs in a house that you hope won't burn down. Otherwise, figure out how much resistance you can stand, use the AWG table to determine the resistance per foot, and choose accordingly. At 50 amps for only several inches in a model aircraft that isn't carrying your own personal body, #12 is likely big enough. Do the wires get hot to the touch? If not -- slightly warm is probably OK -- I wouldn't worry.
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· -- Carl, nn5i@arrl.net
I am an I/C tech. and thanks to company changes I have had to become an electrician.( I have the license ). 12GA wire is rated for 20 amps MAX, 14GA for 15amps. This rating is UL and at 100% duty cycle.
12Ga. wire would carry 50 amps, But not for long. The duty cycle would be very limited.
_______________$WMc%_____________
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The Truth is out there············································ BoogerWoods, FL. USA
Typically, it's not a problem. But you need to consider the specs of the regualator feeding those rails to know for sure. Some have ranges of stability that include not only capacitance, but also equivalent series resistance (ESR). What you need to be most aware of, though, is that bulk capacitance is not a panacea. The localized distribution of that capacitance among the various onboard components is equally important.
-Phil
A lot of devices have a /CS input that has to go low to start a transaction. These won't work at all if /CS is tied high.
Many devices (as you've noted) require a false to true transition on CS or /CS to start a transaction. You can't tie it to true.
This is a wire chart used in aviation. A 12 gauge wire in free air (not in a conduit or in a bundle) at 10 ft in length will handle 50 amps at 14 volts while dropping 1 volt. At 30 feet, only 15 amps. The other chart shows that for a .5 volt drop at 14 volts you would be limited to 8 feet at 50 amps.
Rich H
3. Beside Phil considerations, with big capacitances you should consider also the initial charge current when the circuit is first powered on.
4. Yes/No: On SSTL signaling (eg. betweend ddr sdrams and memory controller) they are used for impedance matching of·the bus and parallel bus termination to avoid reflections but we are speaking of high transmission frequencies (at 2.5V or below). In general for propeller use Carl is right. Even here, a series resistor on the line (depending on the project requirements) can help to reduce the radiated EMI.
5. It could be: maybe the two unconnected ic are picking different noise conditions (usually they have high input impedance and so they are noise-sensitive devices). As Carl said ground the unused inputs.
6. The sising tables for cables are generally good but depends on the application. The cable is a resistor (very low but still a resistor): P=I2R. The·dissipated power depends on the resistance of the cable and the current flowing through it. This is also wasted energy (going in heat).·As $WMc% stated you can undersize a cable (specially for short lengths) depending on the duty cycle (you need to give time to it to cool-down). Some times the limit becomes the voltage drop on the cable, not the current, so they are oversized (tipically on long runs).
Design current and protective devices such as fuses or in the case of mains supply, breakers determine size selected to start with and the above including resistivity formulae determine whether the selected cable is adequate for the application.
don't worry, the same in Italy and I am sure in all the Europe as country regulations should apply the eu comunity regulations (all childs of the same mother). I face with them daily in industrial environment at work but I think SRLM do not have such needs and even you, if you need a quick extension cord at your home, I think you grab the first 5 meters of any 3G2.5 assemble it with a 16A female and male plugs and it's ok (or you compare the datasheets of the cables to have the best one that suit the application?).