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3V <-> 5V Level shifter ?

Started by Martial Chateauvieux January 28, 2005
"Carsten" <xnews1@luna.kyed.com> wrote in message
news:c6inv0d1vli1m8kh5gfjkjc9mopd10lcbt@4ax.com...
> > >You don't need all that. The TX signal from the u-blox can go right into
the
> >input of the atmega, since everything above 2.5V is seen as '1'. The
signal
> >from the atmega to the u-blox can be brought down to 3V by a voltage
divider
> >with two resistors: 12k and 18k. I've used this in a commercial product
and
> >it runs just fine. > > > >Meindert > > > > Doesn't the Mega need 0.6*VCC as Vih(min) , the datasheet says so .. > 0.6 x 5V = 3V
Correct. In my case, the LV device runs at 3.3V, and there it works. So did the OP really mean 3V or 3.3V? But even with 3V it can be done with a shottky diode (to make the 3V output behave like an open collector/drain) and a pull-up resistor. No need for fancy circuits. Meindert
On Sun, 30 Jan 2005 14:39:36 +0100, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:

>> Doesn't the Mega need 0.6*VCC as Vih(min) , the datasheet says so .. >> 0.6 x 5V = 3V > >Correct. In my case, the LV device runs at 3.3V, and there it works. So did >the OP really mean 3V or 3.3V? >But even with 3V it can be done with a shottky diode (to make the 3V output >behave like an open collector/drain) and a pull-up resistor. No need for >fancy circuits. > >Meindert
Sounds interresting , Meindert would you please give an example , i'm a bit of a electronics beginner ??? Im going to interface a Dataflash to a Mega162 , and was planning on using a HCT as levelconverter for the dataflash SO , but would like to avoid it if possible , i will be using a Micrel mic2920 LDO as 3v3 regulator , or maybe "just" the 3 x 1n4148 in series from 5v "trick". Does it have to be a Schottky diode ?? TIA Carsten
"Carsten" <xnews1@luna.kyed.com> wrote in message
news:75qrv0tmarj5ak286vlurgva5sd66lo1pr@4ax.com...
> On Sun, 30 Jan 2005 14:39:36 +0100, "Meindert Sprang" > <mhsprang@NOcustomSPAMware.nl> wrote: > Sounds interresting , Meindert would you please give an example , i'm > a bit of a electronics beginner ??? > Im going to interface a Dataflash to a Mega162 , and was planning on > using a HCT as levelconverter for the dataflash SO , but would like to > avoid it if possible , i will be using a Micrel mic2920 LDO as 3v3 > regulator , or maybe "just" the 3 x 1n4148 in series from 5v "trick". > > Does it have to be a Schottky diode ??
Assuming the dataflash is 3.3V and the mega162 is on 5V, you can connect the DO from flash directly to the DI from the mega. Using a schottky diode is good for two reasons: the voltage drop is lower ( and therefore the raise of the 0-level voltage) and they are much faster and have a lower recovery time. But if the flash is on 3.3V you don't need a diode at all to simulate an open drain output to the mega. The DO from the mega should be brought down to 3.3V. I have used a voltage divider for that (2 resistors) but this was for a low speed connection (57600 baud max.) I am afraid that the signal edges could be affected too much to run this on 1 mbps or higher when communicating to the flash. Meindert
Carsten wrote:
> "Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote: > >>> Doesn't the Mega need 0.6*VCC as Vih(min) , the datasheet says >>> so .. 0.6 x 5V = 3V >> >> Correct. In my case, the LV device runs at 3.3V, and there it >> works. So did the OP really mean 3V or 3.3V? >> But even with 3V it can be done with a shottky diode (to make >> the 3V output behave like an open collector/drain) and a pull-up >> resistor. No need for fancy circuits. >
... snip ...
> > Does it have to be a Schottky diode ??
Or germanium :-) The point is that the lower forward drop does less harm to the low logic level. -- "If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson
On Mon, 31 Jan 2005 09:34:41 +0100, "Meindert Sprang"
<mhsprang@NOcustomSPAMware.nl> wrote:

>The DO from the mega should be brought down to 3.3V. I have used a voltage >divider for that (2 resistors) but this was for a low speed connection >(57600 baud max.) I am afraid that the signal edges could be affected too >much to run this on 1 mbps or higher when communicating to the flash.
Im quite sure that Atmel says that the DataFlash has 5v tolerant input signals. But i have seen the divider trick used on a mmc card. Just for learning the opencolector stuff , how do you connect the schotky and the resistor. I guess the resistor goes to vcc , but what about the diode ?? Is it like this : ---------------|--------Vcc | Pullup | DO----------|>-------|-----------------------AtMega-Input Regards Carsten
"Carsten" <xnews1@luna.kyed.com> wrote in message
news:gp1tv0pc9p8u7h0uefcagrt2i9fo00s0i6@4ax.com...
> On Mon, 31 Jan 2005 09:34:41 +0100, "Meindert Sprang" > <mhsprang@NOcustomSPAMware.nl> wrote: > > I guess the resistor goes to vcc , but what about the diode ?? > > Is it like this : > > ---------------|--------Vcc > | > Pullup > | > > DO----------|>-------|-----------------------AtMega-Input
Almost. The diode must be the other way around and the resistor must be sitting between the atmega input and VCC But you'll probably don't need this at all. Meindert

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