5V to 3.3V UART conversion

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

MAX3390

<omalley.ronan@gmail.com> wrote in message 
news:1140614413.511142.35820@g47g2000cwa.googlegroups.com...
> 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
> 

<omalley.ronan@gmail.com> wrote in message
news:1140614413.511142.35820@g47g2000cwa.googlegroups.com...
> 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.

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

"Bo" <bo@cephus.com> wrote in message
news:c6134$43fc65fc$18d6ec55$1463@KNOLOGY.NET...
> MAX3390

Overkill.

Meindert

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?

omalley.ronan@gmail.com wrote:
> 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?
> 

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

Paul Burke

Hi,

You can use a member of the 74LVCxx familly.

Regards,
Yvan

-- 

******************************
          YBDesign
  http://www.ybdesign.fr
******************************


"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> a &#4294967295;crit dans le message 
de news: hf2pv1hje0eq9h5d0rphebgmqqe3po5086@4ax.com...
> On 22 Feb 2006 07:35:31 -0800, the renowned "omalley.ronan@gmail.com"
> <omalley.ronan@gmail.com> wrote:
>
>>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?
>
> 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 

On 22 Feb 2006 07:35:31 -0800, the renowned "omalley.ronan@gmail.com"
<omalley.ronan@gmail.com> wrote:

>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?

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

Spehro Pefhany wrote:

> 
> 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. 

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

omalley.ronan@gmail.com <omalley.ronan@gmail.com> wrote:
> 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??


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 ----------