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Wiring ForumWiring HardwareHow to get it? › Modifying a board for 3.3 Volt operation.

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Modifying a board for 3.3 Volt operation. (Read 4037 times)
12/18/06 at 00:45:13

Alan_Kilian   Offline
Full Member
Wiring? It looks like
Java to me.

Posts: 118
I recently encountered a situation where I need to talk to a 3.3 Volt chip using an I2C interface.
Since I2C is bidirectional, it's a pain to build a level converter, so I decided to try and see if a
Wiring board would run at 3.3 Volts.

First off, everything says you can't do this.
  • The Atmega-128 won't run at 3.3 Volts
  • The low-voltage Mega-128s with the -L suffix won't run at 25 MHz
But what the heck right?

I hooked the board to a variable voltage power-supply and booted it at 5 Volts with the blinking-
LED demo running. As I lowered the voltage, it stopped running at 4.0 Volts. This is because
the brown-out detector bit is turned on in the fuses, and it's set for 4.0 Volts, so it works.

I reset the fuses , turning off the brownout detector, and it ran all the way down to about 2.8 Volts.

So I built a 5 Volt to 3.3 Volt regulator and hooked it up to J1 and powered the unit from the USB.

After modifying the PCB a little bit to get all the voltages at the right locations, I could still run the
LED demo, but the USB wouldn't run. After some more looking, I found out the FTDI chip was
being held in reset-state and I moved its reset pin to the +5 Volt side and everything works now.

I can reprogram the chip and run programs properly.

I will try and talk to the 3.3 Volt camera chip over the I2C bus next and I'll let you know how it comes out.

Here's a modified Schematic with notes on what I changed.

Here's an image of the modified board.
Yuo can see where I replaced Resistor #4 with a regular one and where I cut the PCB trace near pin #4 of the FTDI chip.

I can't gurantee this will work for you, and it shouldn't work for me, but if you want to experiment, give it a whirl.
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