Reply by Meindert Sprang●February 22, 20062006-02-22
<omalley.ronan@gmail.com> wrote in message
news:1140622531.800329.256600@g47g2000cwa.googlegroups.com...
> 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?
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
> 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 ----------
Reply by Paul Burke●February 22, 20062006-02-22
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
Reply by Spehro Pefhany●February 22, 20062006-02-22
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
Reply by Yvan BOURNE●February 22, 20062006-02-22
Hi,
You can use a member of the 74LVCxx familly.
Regards,
Yvan
--
******************************
YBDesign
http://www.ybdesign.fr
******************************
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> a �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
Reply by Paul Burke●February 22, 20062006-02-22
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
Reply by omal...@gmail.com●February 22, 20062006-02-22
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?
Reply by Meindert Sprang●February 22, 20062006-02-22
"Bo" <bo@cephus.com> wrote in message
news:c6134$43fc65fc$18d6ec55$1463@KNOLOGY.NET...
> MAX3390
Overkill.
Meindert
Reply by Meindert Sprang●February 22, 20062006-02-22
<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
Reply by Bo●February 22, 20062006-02-22
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
>