I've got some older Pro Mini boards, with an ATmega168 operating at 3.3V. When I want to program it using another Arduino, I suppose that the signals should not exceed Vcc, right?
Since my Arduino Uno works with 5V, I tried to power the Mini with 5V on Vcc. This seems to work, tested with the Blink sketch. Since I want to use I2C for multi-processor communication later, I have to stay with that voltage. Is this okay, even for a longer time? Should I shorten the on-board voltage regulator (connect VCC and RAW)?
Later on I noticed that the Serial didn't work correctly, it looks like the baudrate differs with Vcc. When I program and run the board for 5V, the baudrate is okay (9600), but when I program for 3.3V, the effective baudrate is doubled (19200). How that?
Do I have to program for the Vcc used later, in order to obtain proper timing and baudrate? The IDE Board selection indicates 16MHz with 5V, and 8MHz with 3.3V. I'd thought that the clock frequency depends on the quartz, and should be independent from Vcc?
The Mini has a 3.3V regulator. When you power with 5V to Vcc, the regulator will attempt to drop the voltage to the uC Vcc. If you power 5V to RAW then the regulator is reversed and what happens will depend on what the Chinese used for a regulator... From nothing to Poof. On my units, there is a schottky diode that ensure nothing ugly happens.
It is a rather cruel thing to use 5V digital signals on 3.3V powered logic unless the uC pins are 5V tolerent ... Hint, the 386 @ 3.3V is not.
The voltage for an ATmega328P chip can be 2.7V (or less) to 5.5V. It is independent of the speed. However, it can run 16MHz at 5V, but running 16MHz at 3.3V is below the specifications. To run at 16MHz the power needs to be a little more spicy. That's all.
That is why there are two flavours : 5V @ 16MHz and 3.3V @ 8MHz.
But you can run the 8MHz version at 5V as well. You have to select the 3.3V 8MHz board in the Arduino IDE.
Do you have a ATmega168 at 16MHz ? with 3.3V regulator ? That is odd, because that is below the specifications. You can apply 5V to it, but I would remove the 3.3V regulator completely.
Put a big drop of solder on both sides of the regulator and it should fall off. The heat in the big drop should be enough to melt the smd-solder of the regulator.
mrburnette:
The Mini has a 3.3V regulator. When you power with 5V to Vcc, the regulator will attempt to drop the voltage to the uC Vcc. If you power 5V to RAW then the regulator is reversed and what happens will depend on what the Chinese used for a regulator... From nothing to Poof. On my units, there is a schottky diode that ensure nothing ugly happens.
It's all quite confusing
Obviously the Arduino "Mini" and the "Pro Mini" differ, the Pro Mini is said to come with an ATmega168 and in 3.3 and 5 V models. After collecting more information, my Pollin board claims to be a "Pro Mini", to be powerd by 5V at VCC, and to have an on-board 3.3V regulator, but doesn't mention the purpose and connections of that regulator.
After some more experiments and measuring it turned out that the board can operate at both 3.3 and 5 V, depending on whether 5V is supplied to the VCC or RAW pin. This looks to be conforming to the Arduino Pro Mini, except that the Pollin board doesn't have an power isolation jumper.
I'll ask the manufacturer to confirm or to supply the missing information.
Remains the question about the different baudrate. I cannot test the 3.3V operation right now, because I don't have a 3.3V TTY. The Blink sketch shows the same blink rate, regardless of 3.3 or 5V operation.
You better talk about 8MHz and 16MHz models.
Every 8MHZ model can operate at 3.3V and 5V. The same sketch will do the same thing.
Every 16MHz model can operate at 5V, and maybe also at 3.3V (but 3.3V is a little low to run 16MHz).
About the confusing part:
When the voltage regulator output is 3.3V and you apply 5V to the output of the voltage regulator, what will happen ? We don't know. That's why I advice to remove that voltage regulator.
Peter_n:
About the confusing part:
When the voltage regulator output is 3.3V and you apply 5V to the output of the voltage regulator, what will happen ? We don't know. That's why I advice to remove that voltage regulator.
Pro Minis (as in the genuine Sparkfun ones) have a shorting pad to do exactly that. Just cut the short trace (unlabeled but located next to the ground pin next to pin 2) to remove the regulator and power LED from the circuit entirely.
evildave_666:
Pro Minis (as in the genuine Sparkfun ones) have a shorting pad to do exactly that. Just cut the short trace (unlabeled but located next to the ground pin next to pin 2) to remove the regulator and power LED from the circuit entirely.
Please see post from CrossRoads below.. I completely screwed up my pins on this one! Slap me; I know better!
On many Clones.... there is a Vcc input and there is a Raw (+) input. The Raw Vcc input bypasses the regulator and is for low-power operation using a stable 5V (for 16MHz) to the uC. Many of the 5Volt clones can run at 3.3V on the Raw Vcc input, the regulator not being utilized. For normal room temperature spans, I have never had a 328P 16MHz Pro Mini to not work at 3.3V from a quality, low-impedance supply. Obviously, Atmel states this is outside the normal, but if you like to play around, this is a cheap way to use 3.3V sensors without voltage conversion. What I do not know is what happens to a 3.3V regulator on an 8MHz board if one applies 5V to the Raw pin. Maybe nothing, maybe something ugly.
RAW is the input to the regulator.
VCC is the output of the regulator, connected to the uC VCC input pins.
The Micrel brand regulator on original 16MHz Prominis is tolerant of 5V applied to VCC with nothing connected to RAW.
The Micrel brand regulator on original 8MHz Prominis is tolerant of 3.7V-4.2V from a LiPo applied to VCC with nothing connected to RAW. If the processor had a 16MHz bootloader installed, but was run at 8 MHz instead, then things like Serial would need to run at double-speed to work normally (such as setting Serial.begin(19200) to work a t 9600.
Clones, who knows what regulator is installed, as pointed out.
5V on RAW input should be enough for many regulators to make a good 3.3V output. Most only need Vout + 1V on the input. http://www.micrel.com/_PDF/mic5205.pdf
