5V to 3.3V UART conversion

Feb 22, 2006 10 Replies
O

Hey,



I am trying to link the 5V serial port on my ADuC831 (8051) to the 3.3V



serial port on my Analog Devices DSP board. Transmission is one way only (5V -> 3.3V). I have no idea what to do to drop the voltage.


I read somewhere that it should simply be a case of adding a series resistor, the value of which could be calculated by finding out the max



current from the data sheets of both boards. However I haven't been able to find any current values on the data sheets.


Should I use a resistor, transistor, a diode maybe??


Greatful for all suggestions!


Thanks


Ronan



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M

This will be unreliable, since the input resistances can vary. Use a series resistor AND a resistor to ground on the 3.3V side. In several designs of mine, I use 12k in series, followed by 18k to ground (assuming CMOS compatible inputs). Otherwise use lower values wih the same ratio. Even 1k2 and 1k8 will work. Works like a charm.

Meindert

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O

but what happens if there is a spike in the voltage on the 5v side of the voltage divider? My dsp board cant tolerate anything greater than

3.8V max. is the voltage divider really that reliable?
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P

If there's a spike on the 5V side that will kill the DSP, it will almost certainly kill the 8051 too.

Paul Burke

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Y

Hi,

You can use a member of the 74LVCxx familly.

Regards, Yvan

--

****************************** YBDesign
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"Spehro Pefhany" a écrit dans le message de news: snipped-for-privacy@4ax.com...

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S

Usually the source voltage is not that important if the current is limited (there is a protection network between the input and Vdd). Yes, it can be reliable. You might get increased power consumption from going the other way, BTW.

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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P

I've fallen over by simply using a resistor between a 5V system and a

3.3V system... the small current, combined with the almost zero load on the 3.3V side, was enough to drive the 3.3V up to about 4V- the regulator couldn't sink. No damage done, but the flash on the MSP430 wouldn't program, and red face when I had to add a zener to make it work.

Paul Burke

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U

Look for the SN74CBTD Series switches.

Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
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M

The junction diodes on the 3.3V input to the VCC/GND will take care of that, while the current will be limited by the series resistor.

Meindert

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