I want to run an Xbee via the 3.3v pin in order to perform wifi functions, a Deadon RTC from 3.3v, a buzzer from the 5v and a TLC pwm IC from 5V.
I want to know if the GNDs need to be connected between these devices or if there should be anything between them.
Thanks,
Endevor:
I want to know if the GNDs need to be connected between these devices or if there should be anything between them.
Only if you want them to work.
For the I2C devices at 3.3V, you need a level shifter.
Would I need to if I use a Due over a Mega? Or would I need that for the 5V instead of the 3.3v?
Would I need to if I use a Due over a Mega? Or would I need that for the 5V instead of the 3.3v?
You haven't mentioned anything so far that couldn't be handled with a 3.3V Pro-Mini. (only stipulation is
that since it doesn't have onboard USB-to-serial you would need an FTDI BASIC to upload sketches).
In addition, there is no reason you can't use an UNO, (the Sainsmart UNO-R3 has a 3.3V -5V switch to set the output voltage of signals, which only works if all your out-going signals are the same voltage.) however that would require the addition of an LLC (Logic Level Converter) . For some strange reason, Newbies seem to have some kind of instinctual fear of LLCs and will do anything to avoid them which the rest of us find amusing because there are so many cheap ics that are plug & play:
LLCs :
1- TXB-0108 (8-BIT) because it is BI-DIRECTIONAL
4-BIT BI-DIRECTIONAL IC
2- 74LVC245 (8-BIT UNI-DIRECTIONAL IC)
3- 74HC4050E
4- Sparkfun Bi-Directional discrete LLCs Breakout Bd.
5- GOOGLE SEARCH for ARDUINO /LLC
The basic premise is simple. The UNI-DIRECTIONAL ICs have one way gates . They are all rated for
either voltage (3.3V or 5V), so you if you have signals that need to go one direction and other signals
that need to go the other direction, you CANNOT combine them in the SAME IC ! This is obvious because you have to connect the power pin for the chip to the voltage level used by the RECEIVER of the signal.
Once you connect that , ALL 8 GATES ON THAT CHIP CAN ONLY BE USED TO SEND to the uC that runs
on that voltage. This means you need TWO of these, one running on 3.3V to send data to your 3.3V
devices, and one running on 5V to send data to your 5V devices (from the 3.3V devices).
The Bi-directional chips are designed for bi-directional interfaces, not one way interfaces.
There are very few applications where you need a bi-directional interface with an arduino.
One example would be using port reads and writes with a bi-directional port on an EEPROM or another uC. Most arduino users aren't doing that. I have a TXB-0108 and it seems to work for one way level shiftiing but if you think about it, it is a waste of technology to use a bi-directional LLC for a uni-directional application. I read the info on the Adafruit page it they recommend it or bi-directional applications, and recommend the 74HCT4050 or 74LVS245 for the uni-directional applications.
The reason Newbies are so terrified of LLC ics is that
they are paralyzed by the fear that they will get the wiring mixed up and blow stuff up. If you are
organized, and you take notes and you make a list of all the signals that need to go from 3.3V to 5V,
using the device-pin #, and another list of all the signals that need to go from 5V to 3.3V , then all
you have to do is wire up two UNI-DIRECTIONAL CHIPS , each one dedicated for ONE of the TWO
DIRECTIONS.
To Review
The Uni-directional LLC chip should be powered by the voltage level of the receiving device because the
Vcc pin on it sets the OUTPUT VOLTAGE (GOT THAT ?) If you are sending from a 3.3V device (like a
xbee) to a 5V device (like an ARDUINO UNO), then the LLC chip would be powered by 5V.
If you are going the OTHER DIRECTION, then obviously you would want it powered by 3.3V.
I shouldn't have to say this, but if it is powered by 5V, it can receive 3.3V signals. If it is powered by 3.3V, if can receive 5V signals, because it is , after all a Logic Level Converter)
If you want to buy a DUE because it runs on 3.3V and saves you the trouble, so be it. I personally , wouldn't care one way or the other because we are only talking about two ics and maybe 20 wires. (if only some of the gates are used 2 power wires per chip, +2 wires per gate x 4 gates on one chip for one direction and 4 gates on the other chip for the other direction= (8+2)+(8+2)=16+4=20). Of all the options in the list I think option #4 (with headers added) would probably be the best choice because each device can be used for either direction. The point to be aware of is that when they say "Bi directional" , they mean hardwired one way or the other. You can choose. Options #2 & #3 are all one way or the other. No choice. The Sparkfun bd is not truly bi-directional like the Adafruit TXB-0108, which can change direction automatically. The Sparkfun bd just lets you use any combination hard-wired one way or the other. After wiring it , it cannot change direction like the TXB-0104 or TXB-0108.
If 20 wires sounds like too many for you, then maybe you are in the wrong hobby. Assuming the TXB 0108 works just as well for uni-directional applications it may be more convenient and cost effective than separate chips for each direction. I would post a question on Adafruit's customer Support Forum first to confirm that is the case. I can't think of any reason why it wouldn't be.
If I didn't have room for that then yeah I would use a Pro-Mini unless there was a compelling reason to get a DUE. It really depends on your application.