Go Down

Topic: 5V where you might not expect it (Read 3 times) previous topic - next topic

pico

Dec 09, 2012, 02:02 pm Last Edit: Dec 10, 2012, 06:21 pm by pico Reason: 1
Got a Due today, started playing with it, and put a custom shield on it that was designed to be powered by either 3v3 or 5V from the ICSP header. Assumed the Vcc on the ICSP header would be 3v3, so that's what I set the voltage selection jumper to on the shield.

Just before I plugged it all in, I had a thought that I should really check what voltage (if any) I was getting from the ICSP header (well, really SPI header since it doesn't do ICSP, but you know what I mean.)

So got the multimeter, and guess what? 5V. Surprised, but saved me letting out a bit of the ol' blue smoke.

AFAIK, ICSP doesn't specify 5V, so not sure why this pin would be connected the 5V rail rather than the 3v3 rail. Only reason I could think of was to make it more idiot-proof in case someone decided to try programming the Due with a ICSP programmer, and might blow it up by putting 5V on the 3v3 rail via the programmer.

I don't know -- maybe there's a better reason. But in any case be warned -- look before you leap on a hybrid 5V/3v3 board!
WiFi shields/Yun too expensive? Embeddedcoolness.com is now selling the RFXduino nRF24L01+ <-> TCP/IP Linux gateway: Simpler, more affordable, and even more powerful wireless Internet connectivity for *all* your Arduino projects! (nRF24L01+ shield and dev board kits available too.)

CrossRoads

Not sure why you were surprised - the Due schematic shows +5V on the SPI and ICSP headers.
There is no jumper/switch on the board to select a voltage that I have seen, is there?
http://arduino.cc/en/uploads/Main/arduino-Due-schematic.pdf
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

pico


Not sure why you were surprised - the Due schematic shows +5V on the SPI and ICSP headers.


Because I hadn't checked the schematic. I had blithely assumed it would be 3v3, since that's what you'd normally expect on the ICSP header on a "pure" 3v3 board. Lucky for me I checked the assumption at the last step, though.

Can you think of a reason why in this case they might have connected it to the 5V rail instead of the 3v3?

In any case, I thought I'd post this in case anyone else might be as foolish as me.
WiFi shields/Yun too expensive? Embeddedcoolness.com is now selling the RFXduino nRF24L01+ <-> TCP/IP Linux gateway: Simpler, more affordable, and even more powerful wireless Internet connectivity for *all* your Arduino projects! (nRF24L01+ shield and dev board kits available too.)

CrossRoads

It does seem an odd situation.  5V is made available on a header, from a boost regulator even, with 4 other signals that are 3.3V level parts and
have a limitation of "Voltage on Input Pins with Respect to Ground...... ..............................-0.3V to + 4.0V"
http://www.atmel.com/Images/doc11057.pdf

One would have thought a part such as TXB0104 for signal level translation might have been standard here to avoid any damage from 5V signals on the header.

Many parts still run on 5V, so it's nice having a source, but the parts need to return 3.3V signals. Connecting up a MAX7219 for instance, 4V-5V is needed for a supply, 3.5V is needed for a minimum high at 5V, so level translation would be needed.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

pico

#4
Dec 09, 2012, 03:13 pm Last Edit: Dec 09, 2012, 03:16 pm by pico Reason: 1
I wonder if they did it this way in consideration of shield compatibility. On an 5V shield, connecting both the 5V power header pin and the Vcc from the ICSP header would be valid, since they are connected on a 5V AVR board. On a Due, this would obviously be problematic if they connected the ICSP header to the 3v3 rail.

I don't know. In any case, the idea of backward shield compatibility was a broken one from the outset. Perhaps this is just a quirky consequence of the implementation of what was basically an irredeemably  bad idea in the first place.
WiFi shields/Yun too expensive? Embeddedcoolness.com is now selling the RFXduino nRF24L01+ <-> TCP/IP Linux gateway: Simpler, more affordable, and even more powerful wireless Internet connectivity for *all* your Arduino projects! (nRF24L01+ shield and dev board kits available too.)

Go Up